Set primitive translations in supercell

        _trans_s
            Translations with respect to supercell basis vectors
        _trans_p
            Translations with respect to primitive cell basis vectors
        _N
            Number of the translations = det(supercel_matrix)

def _set_translations(self):
        """Set primitive translations in supercell

        _trans_s
            Translations with respect to supercell basis vectors
        _trans_p
            Translations with respect to primitive cell basis vectors
        _N
            Number of the translations = det(supercel_matrix)

        """

        pcell = PhonopyAtoms(numbers=[1],
                             scaled_positions=[[0, 0, 0]],
                             cell=np.diag([1, 1, 1]))
        smat = self._supercell_matrix
        self._trans_s = get_supercell(pcell, smat).get_scaled_positions()
        self._trans_p = np.dot(self._trans_s, self._supercell_matrix.T)
        self._N = len(self._trans_s)