code: def get(self, deviceId, measurementId):
          
          record = self.measurements.get(deviceId)
          if record is not None:
              measurement = record.get(measurementId)
              if measurement.recording:
                  device = self.recordingDevices.get(deviceId)
                  device.signalStop()
                  return measurement, 200
              else:
                  return measurement, 400
          return '', 400
docs: signals a stop for the given measurement.
          :param deviceId: the device to measure.
          :param measurementId: the name of the measurement.
          :return: 200 if stop is signalled, 400 if it doesn't exist or is not running.