Return a dict with GO group headers as keys and section(s) as values.

def _init_hdrgo2sections(self):
        """Return a dict with GO group headers as keys and section(s) as values."""
        hdrgo2sections = cx.defaultdict(list)
        for section_name, hdrgos in self.sections:
            for hdrgo in hdrgos:
                hdrgo2sections[hdrgo].append(section_name)
        return hdrgo2sections