cell(ri, ci) {
        return this.sheet().cell(this._minRowNumber + ri, this._minColumnNumber + ci);
    }
Gets a cell within the range.
@param {number} ri - Row index relative to the top-left corner of the range (0-based).
@param {number} ci - Column index relative to the top-left corner of the range (0-based).
@returns {Cell} The cell.