hasCell(columnNumber) {
        if (columnNumber < 1) throw new RangeError(`Invalid column number ${columnNumber}. Remember that spreadsheets use 1-based indexing.`);
        return !!this._cells[columnNumber];
    }
Check if the row has a cell at the given column number.
@param {number} columnNumber - The column number.
@returns {boolean} True if a cell exists, false otherwise.
@ignore