Return a flat list of grouped and sorted GO terms.

def get_nts_sorted(self, hdrgo_prt, hdrgos, hdrgo_sort):
        """Return a flat list of grouped and sorted GO terms."""
        nts_flat = []
        self.get_sorted_hdrgo2usrgos(hdrgos, nts_flat, hdrgo_prt, hdrgo_sort)
        return nts_flat