- additional distribute/reverseDistribute with specified commsType.
  Improves flexibility.

- distribute with nullValue

- support move construct mapDistribute from mapDistributeBase

- refactor handling of schedules (as whichSchedule method) to
  simplify code.

- renumberMap helper for working with compact sub maps
  and renumberVisit for handling walk-ordered compaction.

COMP: make mapDistributeBase data private

- accessor methods are available - direct access is unnecessary
- mapDistribute : inherit mapDistributeBase constructors

STYLE: use List<labelPair>::null() for schedule placeholders

- clearer that they are doing nothing