Get high GO IDs that are actually used to group current set of GO IDs.

def get_sections_2d_nts(self, sortby=None):
        """Get high GO IDs that are actually used to group current set of GO IDs."""
        sections_2d_nts = []
        for section_name, hdrgos_actual in self.get_sections_2d():
            hdrgo_nts = self.gosubdag.get_nts(hdrgos_actual, sortby=sortby)
            sections_2d_nts.append((section_name, hdrgo_nts))
        return sections_2d_nts