Loads OrbitPopulation from HDF file.

        :param filename:
            HDF file

        :param path:
            Path within HDF file store where :class:`OrbitPopulation` is saved.

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)