minIndex() {
                var v = this.get(0, 0);
                var idx = [0, 0];
                for (var i = 0; i < this.rows; i++) {
                    for (var j = 0; j < this.columns; j++) {
                        if (this.get(i, j) < v) {
                            v = this.get(i, j);
                            idx[0] = i;
                            idx[1] = j;
                        }
                    }
                }
                return idx;
            }
Returns the index of the minimum value
@return {Array}