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/multiphase/interFoam/laminar/damBreakWithObstacle/createObstacle.setSet

@@ -1,3 +1,3 @@
 cellSet c0 new
 cellSet c0 invert
-cellSet c0 delete boxToCell (0.375 0.375 0) (0.625 0.625 0.25)
+cellSet c0 subtract boxToCell (0.375 0.375 0) (0.625 0.625 0.25)

tutorials/multiphase/interFoam/laminar/damBreakWithObstacle/system/topoSetDict

@@ -30,7 +30,7 @@ actions
     {
         name    c0;
         type    cellSet;
-        action  delete;
+        action  subtract;
         source  boxToCell;
         sourceInfo
         {