msgs: Clean up message definitions and descriptions

- Add versioning to interfacing messages
- Add header description
- Add units, frame and range wherever possible
- Add [norm] and @range indentifiers

msg/versioned/FixedWingLongitudinalSetpoint.msg

@@ -0,0 +1,14 @@
+# Fixed Wing Longitudinal Setpoint message
+# Used by the fw_lateral_longitudinal_control module
+# If pitch_direct and throttle_direct are not both finite, then the controller relies on altitude/height_rate and equivalent_airspeed to control vertical motion.
+# If both altitude and height_rate are NAN, the controller maintains the current altitude.
+
+uint32 MESSAGE_VERSION = 0
+
+uint64 timestamp                        # time since system start (microseconds)
+
+float32 altitude  			# [m] Altitude setpoint AMSL, not controlled directly if NAN or if height_rate is finite
+float32 height_rate 			# [m/s] [ENU] Scalar height rate setpoint. NAN if not controlled directly
+float32 equivalent_airspeed 		# [m/s] [@range 0, inf] Scalar equivalent airspeed setpoint. NAN if system default should be used
+float32 pitch_direct 			# [rad] [@range -pi, pi] [FRD] NAN if not controlled, overrides total energy controller
+float32 throttle_direct 		# [norm] [@range 0,1] NAN if not controlled, overrides total energy controller