ENH: optionally eliminate duplicates on hashedWordList construction (issue #375)

- makes it easier to use as a wordHashSet replacement for situations
  where we want to avoid duplicates but retain the input order.

- support construction from HashTable, which means it works like the
  HashTable::sortedToc but with its own hashing for these keys.

- expose rehash() method for the user. There is normally no need for
  using it directly, but also no reason to lock it away as private.

applications/test/mesh/Test-mesh.C

@@ -52,7 +52,6 @@ int main(int argc, char *argv[])
     );
 
 
-    
     Info<< "Cell centres" << nl << mesh.cellCentres() << endl;
     Info<< "Cell volumes" << nl << mesh.cellVolumes() << endl;
     Info<< "Cell shapes" << nl << mesh.cellShapes() << endl;