refactoring, deploy Position and XYGridIterator

- try to use more Position instead of (x,y)
  - create PositionWithDpeth for its specific usage in path finding
- create a XYGridITerator that walk cells from grid X then Y.

java/org/artisanlogiciel/games/maze/WallsProvider.java

@@ -22,4 +22,6 @@ public interface WallsProvider
      *   t
      **/
     short getWalls(int x, int y);
+
+    short getWalls(Position cell);
 }