Return a dict with all user GO IDs as keys and their respective header GOs as values.

def get_usrgo2hdrgo(self):
        """Return a dict with all user GO IDs as keys and their respective header GOs as values."""
        usrgo2hdrgo = {}
        for hdrgo, usrgos in self.hdrgo2usrgos.items():
            for usrgo in usrgos:
                assert usrgo not in usrgo2hdrgo
                usrgo2hdrgo[usrgo] = hdrgo
        # Add usrgos which are also a hdrgo and the GO group contains no other GO IDs
        for goid in self.hdrgo_is_usrgo:
            usrgo2hdrgo[goid] = goid
        assert len(self.usrgos) <= len(usrgo2hdrgo), \
            "USRGOS({U}) != USRGO2HDRGO({H}): {GOs}".format(
                U=len(self.usrgos),
                H=len(usrgo2hdrgo),
                GOs=self.usrgos.symmetric_difference(set(usrgo2hdrgo.keys())))
        return usrgo2hdrgo