_findRangeExtent() {
        this._minRowNumber = Math.min(this._startCell.rowNumber(), this._endCell.rowNumber());
        this._maxRowNumber = Math.max(this._startCell.rowNumber(), this._endCell.rowNumber());
        this._minColumnNumber = Math.min(this._startCell.columnNumber(), this._endCell.columnNumber());
        this._maxColumnNumber = Math.max(this._startCell.columnNumber(), this._endCell.columnNumber());
        this._numRows = this._maxRowNumber - this._minRowNumber + 1;
        this._numColumns = this._maxColumnNumber - this._minColumnNumber + 1;
    }
Find the extent of the range.
@returns {undefined}
@private