Swap i and j rows

        As the side effect, determinant flips.

def _swap_rows(self, i, j):
        """Swap i and j rows

        As the side effect, determinant flips.

        """

        L = np.eye(3, dtype='intc')
        L[i, i] = 0
        L[j, j] = 0
        L[i, j] = 1
        L[j, i] = 1
        self._L.append(L.copy())
        self._A = np.dot(L, self._A)