subscribe to the SUB socket, to listen for incomming variables, return a stream that can be listened to.

def subscribe(self, topic=b''):
        """subscribe to the SUB socket, to listen for incomming variables, return a stream that can be listened to."""
        self.sockets[zmq.SUB].setsockopt(zmq.SUBSCRIBE, topic)
        poller = self.pollers[zmq.SUB]
        return poller