Rename some classes in grid system.

Note: The naming of classes is still confusing. "cell" usually refers to class "Grid", and "grid" usually refers to class "NGrid". But it requires a lot of changes to clean this up.

src/server/game/Grids/Notifiers/GridNotifiers.cpp

@@ -215,7 +215,7 @@ void DelayedUnitRelocation::Visit(PlayerMapType &m)
         if (player != viewPoint && !viewPoint->IsPositionValid())
             continue;
 
-        CellPair pair2(Trinity::ComputeCellPair(viewPoint->GetPositionX(), viewPoint->GetPositionY()));
+        CellCoord pair2(Trinity::ComputeCellCoord(viewPoint->GetPositionX(), viewPoint->GetPositionY()));
         Cell cell2(pair2);
         //cell.SetNoCreate(); need load cells around viewPoint or player, that's why its commented