code: function (level, x, y) {
              var url = null;
              if (level >= this.minLevel && level <= this.maxLevel) {
                  url = this.levels[level].url;
              }
              return url;
          }
docs: Retrieves a tile url
  @function
  @param {Number} level Level of the tile
  @param {Number} x x coordinate of the tile
  @param {Number} y y coordinate of the tile