distance_sensor: msg def: changed to SI units

msg/distance_sensor.msg

@@ -2,11 +2,11 @@
 
 uint32 time_boot_ms 	# Time since system boot (us)
 
-uint16 min_distance 	# Minimum distance the sensor can measure (cm)
-uint16 max_distance		# Maximum distance the sensor can measure (cm)
-uint16 current_distance	# Current distance reading (cm)
+uint16 min_distance 	# Minimum distance the sensor can measure (in SI)
+uint16 max_distance		# Maximum distance the sensor can measure (in SI)
+uint16 current_distance	# Current distance reading (in SI)
 
-uint8 type								# Type from MAV_DISTANCE_SENSOR enum
+uint8 type				# Type from MAV_DISTANCE_SENSOR enum
 uint8 MAV_DISTANCE_SENSOR_LASER = 0
 uint8 MAV_DISTANCE_SENSOR_ULTRASOUND = 1
 uint8 MAV_DISTANCE_SENSOR_INFRARED = 2
@@ -14,4 +14,4 @@ uint8 MAV_DISTANCE_SENSOR_INFRARED = 2
 uint8 id				# Onboard ID of the sensor
 
 uint8 orientation		# Direction the sensor faces from MAV_SENSOR_ORIENTATION enum
-uint8 covariance		# Measurement covariance (cm), 0 for unknown / invalid readings
+uint8 covariance		# Measurement covariance (in SI), 0 for unknown / invalid readings