Three stage autonomous multicopter landing

[MaEtUgR]

Describe problem solved by this pull request
Multirotors hit the ground vertically with land speed MPC_LAND_SPEED during autonomous landing. This speed should ensure there's no damage caused upon touch down but is high enough to not unnecessarily prolong the landing procedure. The user expereince of the vehicle hitting the ground at land speed is not necessarily nice. Since it might see the ground approaching using a distance sensor it could slow down further just for touch down.

Describe @ThomasDebrunner 's solution
This PR adds params for crawl speed and landing altitude 3. In case distance sensor measurements are available, the vehicle slows down to crawl speed at landing altitude 3, for an extra smooth landing. In case no LIDAR data is available, the behavior doesn't change.

Note: If there's no distance sensor it behaves the same as before.

Test data / coverage
We use this change for months on verious systems including big multirotors and VTOLs now.

Additional context
Add any other related context or media.

[ThomasDebrunner]

The biggest implication this has is that the land detector now needs to use the crawl speed to detect landing.
Since the crawl speed is lower, detecting a landing is harder. This makes sense, since the touch down is much softer.

For vehicles without distance sensor, MPC_LAND_CRWL can be set to the same as MPC_LAND_SPEED to have the same beahviour as before.

[ThomasDebrunner]

@MaEtUgR I added another commit that increases the default and minimal crawl speed to 0.3. That's the speed we found to be slow enough while still allowing reliable land detection

[hamishwillee]

@MaEtUgR Are there any documentation impacts of this, specifically in: https://docs.px4.io/main/en/flight_modes/land.html ?

• My guess is that minimally we need to add MPC_LAND_CRWL and MPC_LAND_ALT [1-3] in the parameter list. But would be good to add explanation of what is happening too.
• Does this also apply in a mission landing?
• Are there any other relevant landing "things to discuss"? (i.e. in addition to this?)