Given a list of sections containing GO IDs, get a list of sections w/GO nts.

def get_nts_sections(self, sections, sortby=None):
        """Given a list of sections containing GO IDs, get a list of sections w/GO nts."""
        goids = self.get_goids_sections(sections)
        gosubdag = GoSubDag(goids, self.go2obj)
        return [(sec, gosubdag.get_nts(gos, sortby)) for sec, gos in sections]