Allow Catalyst to be initialized with a separate MPI communicator.

For some simulation codes, only a subset of the MPI processes
may use Catalyst but there wasn't any easy way to do that. This
allows Catalyst to be initialized with a different MPI communicator.

Change-Id: Ib09f446653faf60b25c2524241c348d5ad3b1920
Merge topic '14759_Catalyst_MPI_Comm2' into next

72a5764b Allow Catalyst to be initialized with a separate MPI communicator.