get the prior info groups

        Returns
        -------
        prior_groups : list
            a list of prior information groups

def prior_groups(self):
        """get the prior info groups

        Returns
        -------
        prior_groups : list
            a list of prior information groups

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