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

src/functionObjects/field/fieldValues/surfaceFieldValue/surfaceFieldValue.C

@@ -418,15 +418,12 @@ combineSurfaceGeometry
             // Dimension as fraction of surface
             const scalar mergeDim = 1e-10*boundBox(s.points(), true).mag();
 
-            labelList pointsMap;
-
-            PatchTools::gatherAndMerge
+            Foam::PatchTools::gatherAndMerge
             (
                 mergeDim,
                 primitivePatch(SubList<face>(s.faces()), s.points()),
                 points,
-                faces,
-                pointsMap
+                faces
             );
         }
         else
@@ -444,15 +441,12 @@ combineSurfaceGeometry
             // Dimension as fraction of mesh bounding box
             const scalar mergeDim = 1e-10*mesh_.bounds().mag();
 
-            labelList pointsMap;
-
-            PatchTools::gatherAndMerge
+            Foam::PatchTools::gatherAndMerge
             (
                 mergeDim,
                 primitivePatch(SubList<face>(s.faces()), s.points()),
                 points,
-                faces,
-                pointsMap
+                faces
             );
         }
         else