ENH: direct support for broadcast of bitSet

- the internal data are contiguous so can broadcast size and internals
  directly without an intermediate stream.

ENH: split out broadcast time for profilingPstream information

STYLE: minor Pstream cleanup

- UPstream::commsType_ from protected to private, since it already has
  inlined noexcept getters/setters that should be used.

- don't pass unused/unneed tag into low-level MPI reduction templates.
  Document where tags are not needed

- had Pstream::broadcast instead of UPstream::broadcast in internals

src/Pstream/mpi/UIPstreamRead.C

@@ -77,7 +77,7 @@ void Foam::UIPstream::bufferIPCrecv()
 
     messageSize_ = UIPstream::read
     (
-        commsType_,
+        commsType(),
         fromProcNo_,
         recvBuf_.data(),
         recvBuf_.capacity(),