save potentials to a directory

def _save_potentials(self, directory):
        """save potentials to a directory
        """
        print('saving potentials')
        digits = int(np.ceil(np.log10(self.configs.configs.shape[0])))
        for i in range(0, self.configs.configs.shape[0]):
            pot_data = self.get_potential(i)
            filename_raw = 'pot{0:0' + '{0}'.format(digits) + '}.dat'
            filename = directory + os.sep + filename_raw.format(i + 1)

            nodes = self.grid.nodes['sorted'][:, 1:3]
            all_data = np.hstack((
                nodes,
                pot_data[0][:, np.newaxis],
                pot_data[1][:, np.newaxis],
            ))
            with open(filename, 'wb') as fid:
                np.savetxt(fid, all_data)