By default, ROSS will use MPI_COMM_WORLD for all its communications. tw_comm_set allows the user to change the communicator used by ROSS, for instance to run ROSS on a subset of the ranks of a larger MPI application. tw_comm_set should be called before calling tw_init. If tw_comm_set is used, then the user is responsible for calling MPI_Finalize after calling tw_end.

Note to ROSS developers: the addition of this functionality changes the internal communicator used by ROSS. From now on, MPI_COMM_ROSS should be used instead of MPI_COMM_WORLD.