forEach(callback) {
        for (let ri = 0; ri < this._numRows; ri++) {
            for (let ci = 0; ci < this._numColumns; ci++) {
                callback(this.cell(ri, ci), ri, ci, this);
            }
        }

        return this;
    }
Call a function for each cell in the range. Goes by row then column.
@param {Range~forEachCallback} callback - Function called for each cell in the range.
@returns {Range} The range.