ENH: properly trap any sub-rank changes in allocatePstreamCommunicator

- not currently used, but it is possible that communicator allocation
  modifies the list of sub-ranks. Ensure that the correct size is used
  when (re)initialising the linear/tree structures.

STYLE: adjust MPI test applications

- remove some clutter and unneeded grouping.
  Some ideas for host-only communicators