getTileUrl(level, x, y) {
        if (level === this.maxLevel) {
            return this.url; //for original image, preserve url
        }
        //make up url by positional args
        return `${this.url}?l=${level}&x=${x}&y=${y}`;
    }
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