Need to free comms before mpi finalize

[tclune]

pFUnit creates an MPI communicator for each MPI test case. These are not free'd. This appears to cause some problems in MPI_Finalize() for some MPI flavors/platforms.

To do this, we should add a deferred TPB free() to ParallelContext()`. Each subclass will need to implement the appropriate logic. Then wherever parallel contexts are created, there should be a companion call to free(). It is really not very many places.