getColumn(colIndex) {
        Util.need(colIndex >= 0 && colIndex <= this.width(), "colIndex >= 0 && colIndex <= this.width()");
        let col = [];
        for (let r = 0; r < this._height; r++) {
            col.push(this.cell(colIndex, r));
        }
        return col;
    }
@param {!int} colIndex
@returns {!Array.<!Complex>}