More consistent use of combineReduce, simpler and/or reductions

  - broadcast           : (replaces scatter)
  - combineReduce       == combineGather + broadcast
  - listCombineReduce   == listCombineGather + broadcast
  - mapCombineReduce    == mapCombineGather + broadcast
  - allGatherList       == gatherList + scatterList

  Before settling on a more consistent naming convention,
  some intermediate namings were used in OpenFOAM-v2206:

    - combineReduce       (2206: combineAllGather)
    - listCombineReduce   (2206: listCombineAllGather)
    - mapCombineReduce    (2206: mapCombineAllGather)