Reconcile discrepancies MPI vs. tMPI initialization, acquisition, and finalization. Unify other similar resource management, such as GPU device contexts, thread pool, cpuset, etc. Follow up from #3644 Relates to #3650 - [ ] Merge `gmxapi::MpiContextManager` and `gmx::init`/`gmx::finalize` - [ ] Improve RAII semantics for "MpiContext". Either find a way to join the initialization of thread-MPI context with its acquisition or consider alternatives to using such an object in the corresponding tMPI program scope. - [ ] Clarify the `SimulationContext` invariant(s). Encode the invariant(s) with strong typing to improve readability across tMPI vs. MPI builds. - [ ] Encapsulate the `libraryCommunicator` from the MPI `SimulationContext` into a PhysicalNodeCommunicator. (tMPI can have an unset `std::optional` or similar if such resources cannot be provided through SimulationContext on this code path.) - [x] Python bindings for `gmxapi::ResourceAssignment`; Allow gmxapi to re-wrap an mpi4py communicator for `gmxapi::createContext()` (#4422) # Still deferred #3644 notes that multisim is reasonably a higher-level detail that can be separated into layers, such that `SimultionContext` can abstractly present the simulator with a single communicator or abstract communication resources. We should reserve such discussions for separately tracked issues.