sensors: add baro calibration and cleanup

- sensor_baro.msg use SI (pressure in Pascals)
 - update all barometer drivers to publish directly and remove PX4Barometer helper
 - introduce baro cal (offset) mainly as a mechanism to adjust
relative priority
 - commander: add simple baro cal that sets baro offsets to align with
GPS altitude (if available)
 - create new sensors_status.msg to generalize sensor reporting

msg/CMakeLists.txt

@@ -152,6 +152,7 @@ set(msg_files
 	sensor_preflight_mag.msg
 	sensor_selection.msg
 	sensors_status_imu.msg
+	sensors_status.msg
 	system_power.msg
 	takeoff_status.msg
 	task_stack_info.msg