[Multirotor Only] Automatic learning of hover throttle

[JulioCesarMatias]

This PR adds an algorithm that has the ability to automatically learn the value of the Throttle Hover to be used in flight modes that depend on Alt-Hold (Pos-Hold, WP and RTH). The Throttle Hover value calculated by the algorithm will slowly move towards the average engine power whenever the vehicle is maintaining a constant hover (Functional only when Alt-Hold is active (Alt-Hold alone, or Pos-Hold), does not work on Angle, Horizon or Acro). By default this feature will always be active, and can be turned off by the user through the nav_mc_thr_hover_learn parameter.

Two good things this PR promises:

This prevents users from creating multiple battery profiles, with a different Hover configuration to fly with batteries of different voltages.

The user does not need to fly and find the correct Throttle Hover value to be set in the nav_mc_hover_thr parameter, the algorithm will do it itself.

How to make the feature calculate the Hover value for your Multirotor:

1 - Arm

2 - TakeOff

3 - Fly hovering in Alt-Hold or Pos-Hold.

4 - Avoid vertical speeds (Z) above 60 cm/s up and down (Above that the feature will be turned off).

5 - Avoid making the Roll and Pitch angles greater than 5 degrees of inclination.

6 - Wait for something between 10 to 15 seconds (The time here depends on how your multirotor is behaving, see previous steps 4 and 5).

7 - Disable Alt-Hold or Pos-Hold.

8 - Land

9 - Disarm. Done, you have an automatically calculated Throttle Hover value (See the new value stored in the nav_mc_hover_thr parameter)