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/incompressible/overPimpleDyMFoam/twoSimpleRotors/system/topoSetDict

@@ -59,7 +59,7 @@ actions
     {
         name    c1;
         type    cellSet;
-        action  delete;
+        action  subtract;
         source  cellToCell;
         sourceInfo
         {

tutorials/incompressible/pimpleFoam/RAS/TJunctionFan/system/topoSetDict

@@ -65,7 +65,7 @@ actions
     {
         name    baffleFaceSet;
         type    faceSet;
-        action  delete;
+        action  subtract;
         source  boxToFace;
         sourceInfo
         {

tutorials/incompressible/simpleFoam/pipeCyclic/system/topoSetDict

@@ -63,7 +63,7 @@ actions
     {
         name    c0;
         type    cellSet;
-        action  delete;
+        action  subtract;
         source  faceToCell;
         sourceInfo
         {