import potentials from a directory

def _read_potentials(self, pot_dir):
        """import potentials from a directory
        """
        if self.assignments['potentials'] is not None:
            print('Potentials already imported. Will not overwrite!')
            return
        else:
            self.assignments['potentials'] = {}

        pot_files = sorted(glob(pot_dir + os.sep + 'pot*.dat'))
        for nr, filename in enumerate(pot_files):
            with open(filename, 'r') as fid:
                pot_data = np.loadtxt(fid)

                nids = self.nodeman.add_data(
                    pot_data[:, 2:4],
                )
                # store cids for later retrieval
                self.assignments['potentials'][nr] = nids