Interface updates allowing client to provide MPI communicator.

Interface updates allowing client to provide MPI communicator.

gmxapi::MpiContextManager is acquired earlier in API initialization
and has a larger role in setting up the SimulationContext as a member
of gmxapi::ContextImpl. The MpiContextManager can be acquired with
appropriate default values for the various MPI-related GROMACS build
variants, or it can be initialized with user-provided resources.

gmxapi::Context creation method has been updated to accept optional
non-default resources from the client.

A template header helps MPI-enabled client code to provide resources
to createContext(). The client requires minimal awareness of whether
GROMACS was built for thread-MPI or an installed MPI library.
Thread-MPI details are isolated from any translation unit that might
include the client MPI tool chain.

Refs #3644