cross3(other) {
        Util.need(this.width() === 1 && this.height() === 3, "This isn't a 3d column vector.");
        Util.need(other.width() === 1 && other.height() === 3, "Other's not a 3d column vector.");
        return Matrix.generate(1, 3, r => {
            let [i, j] = [(r+1) % 3, (r+2) % 3];
            let a = this.cell(0, i).times(other.cell(0, j));
            let b = this.cell(0, j).times(other.cell(0, i));
            return a.minus(b);
        });
    }
Computes the cross product of two 3d column vectors.
@param {!Matrix} other
@returns {!Matrix}