cumulativeSum() {
                var sum = 0;
                for (var i = 0; i < this.rows; i++) {
                    for (var j = 0; j < this.columns; j++) {
                        sum += this.get(i, j);
                        this.set(i, j, sum);
                    }
                }
                return this;
            }
Computes the cumulative sum of the matrix elements (in place, row by row)
@return {Matrix} this