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

Added frustmag_config_policy, derived from monomial_config_policy.

New executables for svm-order-params:
* frustmag-learn
* frustmag-test
* frustmag-coeffs

Configure project for use of specific Hamiltonian and lattice, e.g.:

    cmake -DHAMILTONIAN=heisenberg -DLATTICE=kagome ..

Allow for basename with periods (useful with parameter variations and batch
scripting). No longer using the alps::fs functions but relying on C++17's
std::filesystem which is still experimental. Also replaced the ALPS
convenience_params header with a custom version to use those.