STYLE: revert bitSet/HashSet setMany() to set() method (issue #837)

- improves backward compatibility and more naming consistency.
  Retain setMany(iter1, iter2) to avoid ambiguity with the
  PackedList::set(index, value) method.

applications/utilities/mesh/advanced/modifyMesh/cellSplitter.C

@@ -374,7 +374,7 @@ void Foam::cellSplitter::setRefinement
 
         const cell& cFaces = mesh_.cells()[celli];
 
-        faceUpToDate.unsetMany(cFaces);
+        faceUpToDate.unset(cFaces);
     }
 
     forAll(faceUpToDate, facei)