mavros订阅PX4的定高激光以及悬停油门估计数据

借用已有的Altitude这个话题发布定高数据和基础油门估计

备注：v.1.11.3之后的PX4固件才加入了基础油门估计。

在src/modules/mavlink/mavlink_messages.cpp的class MavlinkStreamAltitude : public MavlinkStream函数里面
修改增加部分如下：

	uORB::Subscription _local_pos_sub{ORB_ID(vehicle_local_position)};
	uORB::Subscription _home_sub{ORB_ID(home_position)};
	uORB::Subscription _air_data_sub{ORB_ID(vehicle_air_data)};

	//wzy
    uORB::Subscription _hover_thrust_estimate_sub{ORB_ID(hover_thrust_estimate)};
    uORB::Subscription _distance_sensor_sub{ORB_ID(distance_sensor)};

	/* do not allow top copying this class */
	MavlinkStreamAltitude(MavlinkStreamAltitude &) = delete;
	MavlinkStreamAltitude &operator = (const MavlinkStreamAltitude &) = delete;

以及

		//wzy
        hover_thrust_estimate_s thrust_estimate;
		_hover_thrust_estimate_sub.copy(&thrust_estimate);
		msg.altitude_amsl = thrust_estimate.hover_thrust;
        //wzy
        distance_sensor_s distanceSensor;
        _distance_sensor_sub.copy(&distanceSensor);
        msg.altitude_local = distanceSensor.current_distance;

		// local position timeout after 10 ms
		// avoid publishing only baro altitude_monotonic if possible
		bool lpos_timeout = (hrt_elapsed_time(&local_pos.timestamp) > 10_ms);

		if (lpos_updated || (air_data_updated && lpos_timeout)) {
			msg.time_usec = hrt_absolute_time();
			mavlink_msg_altitude_send_struct(_mavlink->get_channel(), &msg);

			return true;
		}

		return false;

另外，为了增加该话题的发布速度，修改src/modules/mavlink/mavlink_main.cpp的如下部分修改

case MAVLINK_MODE_ONBOARD:
		// Note: streams requiring low latency come first
		configure_stream_local("TIMESYNC", 10.0f);
		configure_stream_local("CAMERA_TRIGGER", unlimited_rate);
		configure_stream_local("HIGHRES_IMU", 50.0f);
		configure_stream_local("LOCAL_POSITION_NED", 30.0f);
		configure_stream_local("ATTITUDE", 100.0f);
//		configure_stream_local("ALTITUDE", 10.0f);
		configure_stream_local("ALTITUDE", 30.0f);//wzy
		configure_stream_local("DISTANCE_SENSOR", 10.0f);
		configure_stream_local("MOUNT_ORIENTATION", 10.0f);
		configure_stream_local("OBSTACLE_DISTANCE", 10.0f);
		configure_stream_local("ODOMETRY", 30.0f);