Load TripleOrbitPopulation from saved .h5 file.

        :param filename:
            HDF file name.

        :param path:
            Path within HDF file where data is stored.

def load_hdf(cls, filename, path=''):
        """
        Load TripleOrbitPopulation from saved .h5 file.

        :param filename:
            HDF file name.

        :param path:
            Path within HDF file where data is stored.

        """
        df_long = pd.read_hdf(filename,'{}/long/df'.format(path))
        df_short = pd.read_hdf(filename,'{}/short/df'.format(path))
        return cls.from_df(df_long, df_short)