ENH: topoSetSource::SUBTRACT enum action (#1060)

- old 'DELETE' enum was easily confused with 'REMOVE', which removes
  the set, not the elements from the set.

- provide corresponding subtractSet() method

STYLE: HashSet set/unset instead of insert/erase methods in topoSetSource

- simplifies switching to/from bitSet storage

tutorials/mesh/stitchMesh/simple-cube1/system/topoSetDict

@@ -56,7 +56,7 @@ actions
     {
         name    inner;
         type    cellSet;
-        action  delete;
+        action  subtract;
         source  cellToCell;
         sourceInfo
         {
@@ -67,7 +67,7 @@ actions
     {
         name    outer;
         type    cellSet;
-        action  delete;
+        action  subtract;
         source  cellToCell;
         sourceInfo
         {