Return a dict with all user GO IDs as keys and their sections as values.

def get_usrgo2sections(self):
        """Return a dict with all user GO IDs as keys and their sections as values."""
        usrgo2sections = cx.defaultdict(set)
        usrgo2hdrgo = self.get_usrgo2hdrgo()
        get_sections = self.hdrobj.get_sections
        for usrgo, hdrgo in usrgo2hdrgo.items():
            sections = set(get_sections(hdrgo))
            usrgo2sections[usrgo] |= sections
        assert len(usrgo2sections) >= len(self.usrgos), \
            "uGOS({U}) != uGO2sections({H}): {GOs}".format(
                U=len(self.usrgos),
                H=len(usrgo2sections),
                GOs=self.usrgos.symmetric_difference(set(usrgo2sections.keys())))
        return usrgo2sections