Altitude Hold Flight Mode

[juangallostra]

Feature / Bug description

It is possible to fly in Altitude Hold Mode. Furthermore, when flying in altitude hold mode:

1. Both the Mosquito 150 and Mosquito 90 exhibit a stable flight and are easy to control.
2. When the throttle stick lies inside the deadband the drone maintains its height.
3. When the throttle stick is moved outside of the deadband a vertical velocity controller regulates the drone's vertical velocity (instead of having a direct input from the throttle stick value).

Notes: There is some work in the altitude-control (https://github.com/BonaDrone/Hackflight/tree/altitude-hold) branch which has not been included yet in the filter-integration or controller-refactor branches. Mainly, the vertical velocity control when the throttle stick is outside of the deadband.

Expected behavior

When flying in Altitude Hold flight mode the drone (both the Mosquito 150 and Mosquito 90) exhibits a stable flight, is easy to control and maintains its height when the throttle stick lies inside the deadband. When the stick is moved outside of the deadband a vertical velocity controller regulates the vertical velocity so that:

1. The drone moves smoothly
2. There is a smooth transition when moving the throttle stick in/out of the deadband.