Fixed-wing attitude controller clean up

[sfuhrer]

Clean up / refactor the FW attitude control classes a bit.

Originally we used to have a fixed-wing "attitude" controller library that also contained the rate control methods. That was then moved to the new fixed-wing rate control module. Currently there is though still the wheel attitude and rate control in it, together with the attitude controller for the in-flight phase. As these controllers are largely different (different inputs etc), I find it cleaner to completely split it up into four classes that don't share a common base class:

RollController
PitchController
YawController
WheelController

Going forward we can then also place the wheel controller at a different place if needed, as it's no longer linked to the attitude controller.

[dagar]

Nice, definitely long overdue.

[Jaeyoung-Lim]

Awesome! Thanks for the long overdue refactor.
Nothing really stands out and looks good to me :)

For moving forward -> Do we want to keep the structure of controllers controlling the three axis independently? Or would it be better to have a full attitude based error metric (e.g. quaternion based like multicopters) for controlling attitude?

[sfuhrer]

Or would it be better to have a full attitude based error metric (e.g. quaternion based like multicopters) for controlling attitude?

Nothing speaks against having a unified one. Ideally even a shared one between MC/FW. We just need to find a good place for the turn coordination controller.