The member types `phase_label` and `phase_sweep_policy_type` formerly
required a definition as part of the TKSVM simulation class interface.
The need for this historically originated in when classifiers and sweep
policies were determined at compile time. Now that these are derived of
abstract policies, the remaining uses of these types could be replaced
by their canonical definitions:

    using phase_label = typename phase_space::classifier::policy<phase_point>::label_type;
    using phase_sweep_policy_type = phase_space::sweep::policy<phase_point>;

TKSVM-compliant simulation classes are thus now longer required to
implement these.

Renamed concept of `LatticePolicy` to `ClusterPolicy` to avoid confusion
with frustmag `Lattice`s.

Define `gauge_config_policy` as an alias to `clustered_config_policy`.

Its configuration(...) function now adheres to the common method of
accessing elements by `cell[block(a)][component(a)]`.

New approach is agnostic w.r.t. client codes. The `sim_base` type needs
to be defined in the main client-code include.

Sized sweep policies allow for the definition of a single
`fixed_from_sweep` classifier that works with points generated from
arbitrary sweep policies.

This is in preparation of making phase classifiers selectable at
runtime.

Classifiers now provide additional functions `name` and `size` to expose
human-readable labels.

As a consequence, the `phase_diagram` classifier could be rewritten such
that a single class covers both D2h and D3h cases, and indeed all phase
diagrams defined on the same (J1J3) parameter space.

Sweep policy used to generate phase space points for the testing stage
can be specified from parameters using `test.policy`. Any other
parameters are specified in much the same way as their `sweep.*`
counterparts. The only difference is that `test.policy` defaults to
"line_scan" (meaning that existing parameter files remain valid),
whereas `sweep.policy` defaults to "cycle".

Also note that the "multipurpose" parameter `sweep.N` (which applied to
the `uniform` and `uniform_line`) has been replaced with separate `N`
parameters.

The change described in the commit message of 489a6f27 has been
implemented. Client codes have to replace any usage of the protected
`measurements` member variable (of `alps::mcbase`) with the
`measurements()` member function. Failing to do so generates errors
(because the function shadows the variable) thus accidental misuse is
precluded.

Previously, the signatures of
* reset_sweeps
* phase_space_point
* update_phase_point
didn't match exactly across all client codes, leading to compiler errors
because of the `override` keyword.

Also, the embarrassing_adapter defines these functions as pure virtual,
forcing their implementation. The pt_adapter on the other hand defines
implementations for these functions. Simulations derived from the
pt_adapter should still override these but make sure they still call
their base class counterparts.

The embarrassing_adapter / pt_adapter are now *base* classes of the main
simulation class, replacing `alps::mcbase`. For now, this change is only
made for the ising and gauge client codes with the embarrassing_adapter;
the frustmag simulation is defunct as of this commit.

This has several advantages:
* simulation classes can call member functions of their base classes. In
  particular when using PT, the simulation can call `negotiate_update`
  directly, eliminating the need for installation of callbacks, etc.
* mcbase's `measurements` object can be replaced / shadowed. This
  mechanism will allow the `pt_adapter` to rebind the measurements
  object when a PT update is carried out.
* The simulation class does no longer need to obtain the MPI
  communicator.
* Each simulation class directly chooses its preferred parallelization
  model by choosing an appropriate base. This eliminates the future need
  for some sort of trait class which detect if parallel tempering is
  supported / required.
* ...

A few technical notes:
* Simulation codes need to make sure they're calling their direct base
  class's save / load functions.
* The embarrassing_adapter directly inherits from `alps::mc::mpiadapter`
  which however requires a non-const reference of the parameters object.
* The embarrassing_adapter needs to shadow the mpiadapters `run`
  function because the mpiadapter was intended to inherit from the main
  simulation class and thus calls `Simulation::fraction_completed()`.
  However, we want this to be a virtual function call into the derived
  main simulation class.

(Because of the last two points, it is probably advisable to not derive
embarrassing_adapter from mpiadapter at all.)
  This would be to be considered a bug in upstream ALPSCore, but is for
  now worked around by changing the ctor signature of all simulation
  classes and adapters to have non-const refs.
*

SVM interface has changed: update_phase_point(...) is now supposed to
return a bool, indicating whether the point has indeed changed (i.e.
fresh thermalization is in order). It should *no longer* call
reset_sweeps itself. This is because it is intended to be used in PT to
update the current phase space point w/o resetting the progress of the
simulation.

As a consequence, the test_adapter is also no longer allowed to reset
the simulation's measurements member; this is now done in reset_sweeps.
Analogously it befalls to the pt_adapter to clear the measurements for
all the slices in reset_sweeps. I.e. this would be the case when the PT
*batch* is changed, but not when the PT update issues a swap of phase
space points.

The training_adapter no longer manages the sweep. The interface function
update_phase_point has been changed such that it accepts concrete phase
space points, rather than generating them from the sweep policy. The
sweep_policy is only used in the sample program and consumed to generate
all the phase space points.

Resuming simulation from the checkpoint in the sample program works now.

The learn program has been stripped down to its SVM optimization part;
no sampling takes place there.

To compare phases to the "infinite temperature" state where all spins
are randomly oriented, a new flag --infinite-temperature, -i has been
introduced to the learn program. When specified, an additional "fake"
phase space point is "sampled" were the configurations are perfectly
random.

To achieve this, the SVM interface for client codes has been expanded to
include a random_configuration() function. This is a breaking change as
client codes have to implement this from now on.

This feature is intended to be used with the fixed_from_cycle classifier
which has been made aware of the "fake" infinity phase point and will
classifier those samples with a distinct label.

This are actually meaningless and prompt compiler warnings.

Instead (though technically unrelated), declared those functions
constexpr.

Also removed some unused argument names to silence the corresponding
warnings.

Previously all ElementPolicies had to provide their version of
`rearranged_index`. This was unnecessary and led to bugs in the past.
DRY

For all projects so far (ising, gauge, frustmag) this is line_scan. Parameters
associated with line_scan are no longer defined nor read in the generic test
program. Future projects may thus define their own sweep policy for testing if
restrictions to sample selection apply.

Derive mono from components, but do not use gauge-specific mono in generic
coeffs and output_contractions program code.

Moved stuff not related to the gauge model to the root directory.

config_policy.hpp holds the abstract config_policy interface, the (reusable)
monomial_config_policy and the dummy block_config_policy.
gauge/gauge_config_policy.hpp holds the lattice_policy and the concrete
gauge_config_policy class, as well as code to instantiate the config_policy from
parameters.

The config policy concept is promoted from the application to the gauge model
specifically to a general utility. Gauge-specific functionality is retained in
the gauge_config_policy class.

Due to API changes, the Ising code is currently broken and will be restored
later.

Minimize the amount of code that is specific to the gauge model and collect the
rest in an reusable intermediate base class `monomial_config_policy`.

Upstream ALPSCore has merged PR #559 which fixed ALPSCore issue #436. Command
line overrides of parameters are possible again.
  * https://github.com/ALPSCore/ALPSCore/pull/559
  * https://github.com/ALPSCore/ALPSCore/issues/436

The previous workaround is no longer necessary and has been removed; confer
  * commit cdcc6b80
  * commit 3722c6e6
  * commit 7eedfea9

Also did from naming / refactoring of parameters:

* "resolve_sites", "bipartite" switches have been phased out in favor of the
  "cluster" parameter which can have three string values:
    - "single" for single-spin lattice-averaged configurations (the default),
    - "bipartite" for two-spin lattice-averaged configurations (for AFM),
    - "full" for configurations w/o lattice average (replacing "resolve_sites")
* "uniaxial" switch has been replaced with "color" which is one of the strings:
    - "mono" for using only the S^n spins in the configuration (was uniaxial==1)
    - "triad" for using all three colored spins (was uniaxial==0)

Some of the classes have been refactored to reflect this new nomenclature.

Allowing for stateful ElementPolicies (rather than relying on compile-time
constants) opens the door for ElementPolicies that depend on the details of
lattice that are chosen at runtime.