docs: def load_hdf(cls, filename, path=''):
          """Loads OrbitPopulation from HDF file.
  
          :param filename:
              HDF file
  
          :param path:
              Path within HDF file store where :class:`OrbitPopulation` is saved.
          """
          df = pd.read_hdf(filename,'{}/df'.format(path))
          return cls.from_df(df)