Support taking MPI communicator from application

[cwsmith]

During library initialization (or some other high-level object creation; i.e., the mesh) the MPI communicator should be passed in from the user/application and used for all calls to MPI associated with that instance/object.

There is a Fortran API here, but it isn't passed into the C++ layer:

polyMPO/src/pmpo_c.cpp

Lines 54 to 58 in 3c0795d

	void polympo_setMPICommunicator_f(MPI_Fint fcomm){
	MPI_Comm comm = MPI_Comm_f2c(fcomm);
	int commSize;
	MPI_Comm_size(comm,&commSize);
	}