- UPstream::globalComm constant always refers to MPI_COMM_WORLD but
  UPstream::worldComm could be MPI_COMM_WORLD (single world)
  or a dedicated local communicator (for multi-world).

- provide a Pstream wrapped version of MPI_COMM_SELF,
  references as UPstream::selfComm

- UPstream::isUserComm(label)
  test for additional user-defined communicators