...using type traits and meta programming. The multiplexer (mux) will
only support the rebind_communicator member function if at least one of
its constituent updates has it. The mux then forward the call only to
those. The `pt_adapter` assumes that the simulation base class supports
rebind_communicator to pass it on; however this mechanism always one to
use the frustmag simulation w/o parallel tempering in which case also no
`mpi::communicator` has to be defined, i.e. the program compiles without
MPI headers and libs.