Add an actor / actor list to the system's actors
        :param actor: Actor | list<Actor>
        :return: self

def add_actors(self, actor):
        """
        Add an actor / actor list to the system's actors
        :param actor: Actor | list<Actor>
        :return: self
        """
        if isinstance(actor, list):
            self.actors.extend(actor)
        elif isinstance(actor, Actor):
            self.actors.append(actor)
        return self