Return displacements

        Returns
        -------
        There are two types of displacement dataset. See the docstring
        of set_displacement_dataset about types 1 and 2 for displacement
        dataset format.

        Type-1, List of list
            The internal list has 4 elements such as [32, 0.01, 0.0, 0.0]].
            The first element is the supercell atom index starting with 0.
            The remaining three elements give the displacement in Cartesian
            coordinates.
        Type-2, array_like
            Displacements of all atoms of all supercells in Cartesian
            coordinates.
            shape=(supercells, natom, 3)
            dtype='double'

def displacements(self):
        """Return displacements

        Returns
        -------
        There are two types of displacement dataset. See the docstring
        of set_displacement_dataset about types 1 and 2 for displacement
        dataset format.

        Type-1, List of list
            The internal list has 4 elements such as [32, 0.01, 0.0, 0.0]].
            The first element is the supercell atom index starting with 0.
            The remaining three elements give the displacement in Cartesian
            coordinates.
        Type-2, array_like
            Displacements of all atoms of all supercells in Cartesian
            coordinates.
            shape=(supercells, natom, 3)
            dtype='double'

        """

        disps = []
        if 'first_atoms' in self._displacement_dataset:
            for disp in self._displacement_dataset['first_atoms']:
                x = disp['displacement']
                disps.append([disp['number'], x[0], x[1], x[2]])
        elif 'displacements' in self._displacement_dataset:
            disps = self._displacement_dataset['displacements']

        return disps