Add function to get the MPI communicator
Ignore the non-serializable MPI communicator when pickling Halos
Merge pull request #154 from OP2/mpi-communicators

MPI communicators

MPI communicators are not serializable, so we need to ignore them when
pickling Halos. When unpickling we use the default PyOP2 communicator.

Also add a function to return the PyOP2 communicator.