FIX: incorrect parameters for IN_PLACE MPI_Gather, MPI_Scatter

STYLE: mark Pstream::scatterList() as deprecated

- this entry point is not directly used anywhere, only the
  scatterList_algorithm backend is actually used.

  The scatterList() routine is misnomer since it actually works like a
  broadcast that skips overwriting the local rank, but only if used in
  combination with the gatherList() manual implementation that uses
  the same walk pattern.