Combine user namedtuple fields, GO object fields, and format_txt.

def get_go2nt(self, usr_go2nt):
        """Combine user namedtuple fields, GO object fields, and format_txt."""
        gos_all = self.get_gos_all()
        # Minimum set of namedtuple fields available for use with Sorter on grouped GO IDs
        prt_flds_all = get_hdridx_flds() + self.gosubdag.prt_attr['flds']
        if not usr_go2nt:
            return self.__init_go2nt_dflt(gos_all, prt_flds_all)
        usr_nt_flds = next(iter(usr_go2nt.values()))._fields
        # If user namedtuple already contains all fields available, then return usr_go2nt
        if len(set(prt_flds_all).difference(usr_nt_flds)) == 0:
            return self._init_go2nt_aug(usr_go2nt)
        # Otherwise, combine user fields and default Sorter fields
        return self.__init_go2nt_w_usr(gos_all, usr_go2nt, prt_flds_all)