- when constructing from a sendMap, can now also specify a linear
  receive layout instead of a localFirst layout

  This will make it easier to reduce some code (#2932)

- add missing interface for simple distribute of List/DynamicList
  with a specified commsType. Was previously restricted to
  defaultCommsType only.

ENH: mapDistribute distribute/reverseDistribute with specified commsType

STYLE: prefer UPstream vs Pstream within mapDistribute