generate_mesh should take in an MPI communicator

[Patrick Farrell]

It's very useful sometimes to be able to have different teams of processors working independently within the same code. To this end, there has been lots of work in DOLFIN to allow mesh constructors to take in custom MPI communicators. Would it be possible to allow the generate_mesh call to take in the MPI communicator to be used in the generation? E.g.

mesh = generate_mesh(geom, resolution, comm)