code: def add_sensor(self, sensorid=None):
          
          if sensorid is None:
              sensorid = self._get_next_id()
          if sensorid is not None and sensorid not in self.sensors:
              self.sensors[sensorid] = Sensor(sensorid)
          return sensorid if sensorid in self.sensors else None
docs: Add a sensor to the gateway.