UPstream::allocateCommunicator
  - with contiguous sub-procs. Simpler, more compact handling, ranks
    are guaranteed to be monotonic

UPstream::commWorld(label)
  - ignore placeholder values, prevents accidental negative values

- make communicator non-optional for UPstream::broadcast(), which
  means it has three mandatory parameters and thus always fully
  disambiguated from Pstream::broadcast().

ENH: relax size checking on gatherList/scatterList

- only fatal if the List size is less than nProcs.
  Can silent ignore any trailing elements: they will be untouched.