code: def save_nd_to_pickle(nd, path='', filename=None):
      
  
      abs_path = os.path.abspath(path)
  
      if len(nd._mv_grid_districts) > 1:
          name_extension = '_{number}-{number2}'.format(
              number=nd._mv_grid_districts[0].id_db,
              number2=nd._mv_grid_districts[-1].id_db)
      else:
          name_extension = '_{number}'.format(number=nd._mv_grid_districts[0].id_db)
  
      if filename is None:
          filename = "ding0_grids_{ext}.pkl".format(
              ext=name_extension)
  
      # delete attributes of `nd` in order to make pickling work
      # del nd._config
      del nd._orm
  
      pickle.dump(nd, open(os.path.join(abs_path, filename), "wb"))
docs: Use pickle to save the whole nd-object to disc
  
      The network instance is entirely pickled to a file.
  
      Parameters
      ----------
      nd : NetworkDing0
          Ding0 grid container object
      path : str
          Absolute or relative path where pickle should be saved. Default is ''
          which means pickle is save to PWD