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/Maps/MapManager.cpp

@@ -292,7 +292,7 @@ void MapManager::DoDelayedMovesAndRemoves()
 
 bool MapManager::ExistMapAndVMap(uint32 mapid, float x, float y)
 {
-    GridPair p = Trinity::ComputeGridPair(x, y);
+    GridCoord p = Trinity::ComputeGridCoord(x, y);
 
     int gx=63-p.x_coord;
     int gy=63-p.y_coord;