ENH: align MPI_Probe non-blocking handling with header description
- the header states that the commsType is non-blocking or not,
but the implementation actually checked for 'buffered' or not.
STYLE: fix some spacing and some documentation
- 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
