ENH: bundle Pstream:: AllGather methods

- bundles frequently used 'gather/scatter' patterns more consistently.

  - combineAllGather     -> combineGather + broadcast
  - listCombineAllGather -> listCombineGather + broadcast
  - mapCombineAllGather  -> mapCombineGather + broadcast
  - allGatherList        -> gatherList + scatterList
  - reduce               -> gather + broadcast (ie, allreduce)

- The allGatherList currently wraps gatherList/scatterList, but may be
  replaced with a different algorithm in the future.

STYLE: PstreamCombineReduceOps.H is mostly unneeded now