ENH: PatchTools::gatherAndMerge with recovery of the globalIndex

- support globalIndex for points/faces as an output parameter,
  which allows reuse in subsequent field merge operations.

- make pointMergeMap an optional parameter. This information is not
  always required. Eg, if only using gatherAndMerge to combine faces
  but without any point fields.

ENH: make globalIndex() noexcept, add globalIndex::clear() method

applications/utilities/mesh/generation/foamyMesh/foamyHexMeshBackgroundMesh/foamyHexMeshBackgroundMesh.C

@@ -727,14 +727,12 @@ int main(int argc, char *argv[])
 
     pointField mergedPoints;
     faceList mergedFaces;
-    labelList pointMergeMap;
     PatchTools::gatherAndMerge
     (
         tolDim,
         primitivePatch(SubList<face>(isoFaces), isoPoints),
         mergedPoints,
-        mergedFaces,
-        pointMergeMap
+        mergedFaces
     );
 
     if (Pstream::master())