get the observation groups

        Returns
        -------
        obs_groups : list
            a list of unique observation groups

def obs_groups(self):
        """get the observation groups

        Returns
        -------
        obs_groups : list
            a list of unique observation groups

        """
        og = list(self.observation_data.groupby("obgnme").groups.keys())
        #og = list(map(pst_utils.SFMT, og))
        return og