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/sensors_status.msg

@@ -0,0 +1,15 @@
+#
+# Sensor check metrics. This will be zero for a sensor that's primary or unpopulated.
+#
+uint64 timestamp # time since system start (microseconds)
+
+uint32 device_id_primary       # current primary device id for reference
+
+uint32[4] device_ids
+float32[4] inconsistency       # magnitude of difference between sensor instance and mean
+bool[4] healthy                # sensor healthy
+uint8[4] priority
+bool[4] enabled
+bool[4] external
+
+# TOPICS sensors_status sensors_status_baro sensors_status_mag