ENH: consistency of HashSet setMany(), insertMany() with packed-list version

- this also provides a better separation of the intent
  (ie, inserting a single value, or inserting multiply values)

applications/utilities/mesh/advanced/splitCells/splitCells.C

@@ -633,10 +633,7 @@ int main(int argc, char *argv[])
         );
 
         cellSet cutSet(mesh, "cutSet", cutCells.size());
-        forAll(cutCells, i)
-        {
-            cutSet.insert(cutCells[i]);
-        }
+        cutSet.insertMany(cutCells);
 
         // Gets cuts across cells from cuts through edges.
         Info<< "Writing " << cutSet.size() << " cells to cut to cellSet "