ENH: extend Pstream gather templates to support combine or assign operation

- eliminates nearly identical code between 'gather' and 'combineGather'

  * Normal gather updates by assigning the result of the binary operation.
  * Combine gather updates by using a binary operator that modifies
    its first parameter in-place

  By-product of this refactoring are these new variants:

      listGather(), listGatherReduce()
      mapGather(), mapGatherReduce()

  that mirror the previously existing ones

      listCombineGather(), listCombineReduce()
      mapCombineGather(), mapCombineReduce()

  except that they use the 'regular' binary operator