TECS: Cleared up integrator-design on throttle, corrected rate-limiting (WIP)

*** WIP - DO NOT MERGE YET ***

This PR shall be the basis for a discussion on the TECS integrator design: Two changes were introduced:

a) The throttle integrator was changed to just be a constant parameter (intuitively) tunable by the user. Question: What was the exact reasoning behind the previous integrator design, and what advantages does it have over a single fixed-but-tunable parameter?
b) The throttle slew-rate constraint was previously only applied to the feed-forward & proportional parts of _throttle_dem. This was changed.
c) some small fixes in comments, etc.

This is only HIL-tested! Before the actual merge, one would for sure need to test-fly this specific commit. However, the modifications in the code itself have been operated on ETHZ-ASL fixed wing UAVs (on an old firmware!) for many months.