sensors: baro aggregator limit publication rate with new parameter SENS_BARO_RATE

[dagar]

@priseborough as discussed, this is a simple change to the barometer aggregator in the sensors hub (sensors/vehicle_air_data) that limits the publication rate of vehicle_air_data (primary baro static pressure, altitude, temperature) to something reasonable (SENS_BARO_RATE default is 20 Hz). If the underlying data exceeds the limit then it's accumulated and averaged until the next allowed publication.

This is a small optimization to limit sending unnecessary data to the estimator. Saves a small amount of work per ekf2 instance, logging, and helps with replay logging. In cases where the sensor_baro publication rate is relatively high (nearly 100 Hz with the ms5611) it reduces the nominal default logging rate (estimator replay enabled) by 8%.

The will be done for magnetometer and airspeed as well.

[dagar]

Similar to the other sensors I've added a separate timestamp_sample to the sensor_baro message. The difference is very small, but this gets us in the habit of using timestamp as uORB metadata and working towards eventually having it populated automated at publication time.

While looking at sensor_baro usage across the code base I found and fixed a simple case in the FW position controller where the first baro was used directly.

[dagar]