TECS: allow for fast descend up to maximum airspeed

[KonradRudin]

Solved Problem

Descending on a fixed wing can be slow when both the altitude and the airspeed need to be regulated.

Fixes #{Github issue ID}

Solution

• If a high altitude descend is required, the throttle is set to minimum and the altitude control is used to control the airspeed. This guarantees, a fast descend while the airspeed is not overshot.
• If the altitude threshold is reached, the normal TECS control is faded in.

Changelog Entry

For release notes:

Feature: Allow descending faster for fixed wing vehicles up to maximum airspeed

Test coverage

• Flight test log here: https://review.px4.io/plot_app?log=bf0a3139-e65c-4449-b772-57a90397c846
  • Testing with multiple max airspeed and altitude error settings.

[KonradRudin]

Comments on the flight tests:

• Generally, the vehicle does not completely reach the maximum airspeed but it is a bit slower (in the flight test reaching 21.5m/s instead of target 22m/s). This is dues to the fact that the altitude control tuning is generally more responsive than the airspeed tuning. So if the airspeed error is fed into the pitch controller, its less responsive but still good enough to do the job. As an alternative to be more responsive one would need to change the airspeed control parameters and pitch integrator, but i wanted to avoid that.
• The altitude error must be tuned, such that the vehicle won't undershoot the target altitude. It can be seen that when increasing the maximum airspeed that happened for a low altitude error parameter. Was fixed by increasing the threshold.

[KonradRudin]

Also for discussion if we really want this commit in https://github.com/PX4/PX4-Autopilot/pull/22479/commits/f3ccd18e15b6ceeaa357c0aca46e82a868d904f1 This would allow for higher values when the altitude and airspeed errors are not distributed evenly. But it helped to have a better performance such that the pitch controller could track the airspeed.

[RomanBapst]

@KonradRudin It looks like the throttle slew rate does not apply when switching in and out of fast descend mode. I think it would make sense to have it.