Fixed-wing Position Control: Split up control methods for VTOL backtransition

[RomanBapst]

Solved Problem

Previously, the method to control a transition to hover was handling both cases when position was available, and when it was not available.
It's cleaner to have the methods separates for once. Other than that, the method which controls the transition for the case when position is not available, now does heading hold, which is better than just demanded 0 roll, which was done previously.

Changelog Entry

Control heading during backtransition when local position is not available.

Test coverage

Currently tested in SITL, following up with plots from real tests.

Context

The figure below is from a simulation, where the vehicle approaches the transition point with a 10m/s crosswind (see large difference between heading and track)
The heading of the vehicle is tracked as the initial heading when local position is lost (when the nav state changes).
The roll angle setpoint is continuous, despite the mode switch. This is because the controller had already compensated for the crosswind when approaching the transition waypoint.

[sfuhrer]

Have you already some flight testing results to share? I'm curious to see if it really makes a difference to control heading in the Descend case or not.

[RomanBapst]

Have you already some flight testing results to share? I'm curious to see if it really makes a difference to control heading in the Descend case or not.

@sfuhrer Yes, the reason I implemented this was actually because the test vehicle flew a turn with the previous implementation, which was just to set roll to 0. I guess it makes a big difference it you have crosswind or not.
Also, I think this is cleaner in general.

[sfuhrer]

Just some minor comments.
Let's clean up the commits or squash merge.

[sfuhrer]

still over flash on v6x but otherwise good to go.