Return children of input gos plus input gos.

def add_children_gos(self, gos):
        """Return children of input gos plus input gos."""
        lst = []
        obo_dag = self.obo_dag
        get_children = lambda go_obj: list(go_obj.get_all_children()) + [go_obj.id]
        for go_id in gos:
            go_obj = obo_dag[go_id]
            lst.extend(get_children(go_obj))
        return set(lst)