Two issues with mpi communicator configuration

From #21:

Two related issues found while investigating, not addressed here:

• h_comm_size is writable on both config objects, but tpb::diag / gdb::diag
  recompute it from mpi_size, so setting it has no effect.
• gdb_diag's docstring says b_comm_size must be 1, which is enforced nowhere and
  unexplained. If true it deserves a reason; if stale it blocks the one dimension that
  would parallelise the determinant list.

Trace the two configuration objects into tpb::diag and gdb::diag, starting with how mpi_size and h_comm_size are handled. Read the gdb_diag docstring and inspect how b_comm_size is used or validated. Done means the writable settings and documentation agree with the implemented communicator behavior, with tests or checks covering the two reported cases.