Beef up the dummy MPI machinery

This PR does a couple things.  It adds MPI_Barrier functionality to `dummympi` and adds a `COMM_WORLD` attribute that behaves like `mpi4py`'s `MPI.COMM_WORLD`.  There's an example in the `dummympi`'s docstring.

I also replaced everywhere that a `DummyMPIComm` to just using `dummympi.COMM_WORLD` instead.  This is a bit better duck typing (since now the MPI communicator from dummympi is a singleton just like the COMM_WORLD from mpi4py and responds correctly to the `is` operator).

This is also needed for the unification of dummy MPIs used in openmm-tests.