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/foamyHexMesh/flange/system/faceSetDict

@@ -18,8 +18,8 @@ FoamFile
 // Name of set to operate on
 name facesToBeRemoved;
 
-// One of (clear | new | invert | add | delete | subset | list)
-action new;
+// One of (add | subtract | subset | clear | new | invert | list)
+action  new;
 
 // Actions to apply to pointSet. These are all the topoSetSource's ending
 // in ..ToFace (see the meshTools library).

tutorials/mesh/foamyHexMesh/flange/system/topoSetDict-background

@@ -23,7 +23,7 @@ FoamFile
 //
 //     // action to perform on set. Two types:
 //     // - require no source : clear/invert
-//     // - require source    : new/add/delete/subset
+//     // - require source    : add/subtract/subset/new
 //     action  new;
 //
 // The source entry varies according to the type of set: