trace() {
        let total_r = 0;
        let total_i = 0;
        let d = this._width*2 + 2;
        for (let i = 0; i < this._buffer.length; i += d) {
            total_r += this._buffer[i];
            total_i += this._buffer[i+1];
        }
        return new Complex(total_r, total_i);
    }
Returns the matrix' trace (i.e. the sum of its diagonal elements, i.e. the sum of its eigenvalues
if it's square).
@returns {!Complex}