Return task health. If None - task is healthy, otherwise - maximum severity of sensors

		:return: None or WTaskHealthSensor.WTaskSensorSeverity

def healthy(self):
		""" Return task health. If None - task is healthy, otherwise - maximum severity of sensors

		:return: None or WTaskHealthSensor.WTaskSensorSeverity
		"""
		state = None
		for sensor in self._sensors.values():
			if sensor.healthy() is False:
				if state is None or sensor.severity().value > state.value:
					state = sensor.severity()
				if state == WTaskHealthSensor.WTaskSensorSeverity.critical:
					break

		return state