fw pos ctrl: auto landing refactor

- landing slope/curve library removed
- flare curve removed (the position setpoints will not be tracked during a flare, and were being ignored by open-loop maneuvers anyway)
- flare curve replaced by simply commanding a constant glide slope to the ground from the approach entrance, and commanding a sink rate once below flaring alt
- flare is now time-to-touchdown -based to account for differing descent rates (e.g. due to wind)
- flare pitch limits and height rate commands are ramped in from the previous iteration's values at flare onset to avoid jumpy commands
- TECS controls all aspects of the auto landing airspeed and altitude/height rate, and is only constrained by pitch and throttle limits (lessening unintuitive open loop manuever overrides)
- throttle is killed on flare
- flare is the singular point of no return during landing
- lateral manual nudging of the touchdown point is configurable via parameter, allowing the operator to nudge (via remote) either the touchdown point itself (adjusting approach vector) or shifting the entire approach path to the left or right. this helps when GCS map or GNSS uncertainties set the aircraft on a slightly offset approach"

msg/position_controller_landing_status.msg

@@ -1,9 +1,3 @@
-uint64 timestamp		# time since system start (microseconds)
-
-float32 horizontal_slope_displacement
-
-float32 slope_angle_rad
-
-float32 flare_length
-
-bool abort_landing				# true if landing should be aborted
+uint64 timestamp # [us] time since system start
+float32 lateral_touchdown_offset # [m] lateral touchdown position offset manually commanded during landing
+bool abort_landing # true if landing should be aborted