Check that hdrgo set is a set of GO IDs.

def _chk_hdrgoids(hdrgos):
        """Check that hdrgo set is a set of GO IDs."""
        goid = next(iter(hdrgos))
        if isinstance(goid, str) and goid[:3] == "GO:":
            return
        assert False, "HDRGOS DO NOT CONTAIN GO IDs: {E}".format(E=goid)