ENH: remove obsolete Pstream functions (#3087)

- the old Pstream::scatter routines (which were largely a misnomer)
  have been superseded by various broadcast routines, but were left in
  the code with #ifndef/#ifdef Foam_Pstream_scatter_nobroadcast
  guards. Now noisily deprecate them, and remove the old manual tree
  communication in favour of MPI broadcast and/or
  serialize/de-serialize with wrapped Pstream::broadcast

- consolidate various gather methods to include the communication
  structure directly. No functional change, but reduces the number of
  methods.

ENH: add parallel guard to UPstream::whichCommunication() method

- returns List::null() as the schedule for non-parallel instead
  of an inappropriate linear or tree schedule

ENH: Pstream::listGatherValues, Pstream::listScatterValues

- like the existing UPstream versions but supporting non-contiguous