splitPanes(xSplit, ySplit) {
        return this.panes({ state: 'split', xSplit, ySplit });
    }
Splits Panes for this sheet.
@param {number} xSplit (Horizontal Split Position) Horizontal position of the split,
in 1/20th of a point; 0 (zero) if none.
@param {number} ySplit (Vertical Split Position) VVertical position of the split,
in 1/20th of a point; 0 (zero) if none.
@return {Sheet} The sheet