Introduce global MPI configuration object
Update Mass 2D MPI demo
Remove properties size, rank on global MPI config
Assure communicators are of type mpi4py.MPI.Comm or convertable
Add unit test setting PETSc MPI communicator
Add MPI API unit tests
Use common helper function _check_comm in MPIConfig and Halo
Fix remaining mpi4py MPI references (use _MPI)
Merge pull request #176 from OP2/mpi-global-config

Introduce a global MPI configuration object.

Adds an object MPI of type MPIConfig to the op2 namespace. This objects
encapsulates the MPI communicator and ensures a communicator is of type
mpi4py.MPI.Comm or can be converted to this type when set.