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/splitCells/splitCells.C

@@ -633,7 +633,7 @@ int main(int argc, char *argv[])
         );
 
         cellSet cutSet(mesh, "cutSet", cutCells.size());
-        cutSet.insertMany(cutCells);
+        cutSet.insert(cutCells);
 
         // Gets cuts across cells from cuts through edges.
         Info<< "Writing " << cutSet.size() << " cells to cut to cellSet "