ENH: refine mapDistribute methods

- 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

src/sampling/meshToMesh/meshToMesh.C

@@ -550,7 +550,7 @@ void Foam::meshToMesh::calculate(const word& methodName, const bool normalise)
         mapDistributeBase::distribute
         (
             Pstream::commsTypes::nonBlocking,
-            List<labelPair>(),
+            List<labelPair>::null(),
             tgtRegion_.nCells(),
             map.constructMap(),
             false,
@@ -568,7 +568,7 @@ void Foam::meshToMesh::calculate(const word& methodName, const bool normalise)
         mapDistributeBase::distribute
         (
             Pstream::commsTypes::nonBlocking,
-            List<labelPair>(),
+            List<labelPair>::null(),
             tgtRegion_.nCells(),
             map.constructMap(),
             false,