Returns a string of the form 'x, y, z', '-x, -y, z',
        '-y+1/2, x+1/2, z+1/2', etc. Only works for integer rotation matrices

def as_xyz_string(self):
        """
        Returns a string of the form 'x, y, z', '-x, -y, z',
        '-y+1/2, x+1/2, z+1/2', etc. Only works for integer rotation matrices
        """
        xyz = ['x', 'y', 'z']
        strings = []

        # test for invalid rotation matrix
        if not np.all(np.isclose(self.rotation_matrix,
                                 np.round(self.rotation_matrix))):
            warnings.warn('Rotation matrix should be integer')

        return transformation_to_string(self.rotation_matrix, translation_vec=self.translation_vector, delim=", ")