Enhance Linear Descent RTH feature

[MrD-RC]

Addresses an issue/request discussed on Discord

This PR takes the existing Linear Descent option for RTH and enhances it. Firstly, Linear Descent is now no longer an RTH type. If previously using AT_LEAST_LINEAR_DESCENT, you should now just use AT_LEAST as the RTH type. Linear Descent is now a separate option that can be used with any other RTH type. To do this, the target altitude of the descent is now the nav_rth_home_altitude. In some cases, if flying below the home position, this could even be a linear ascent.

An option has also been added to decide how far away the linear descent starts. This is via the nav_rth_linear_descent_start_distance parameter. Current behaviour can be maintained by setting this to 0 [default]. You can also specify a distance from home when the descent begins. This is in metres and be up to 10km. Of course, this value can be adjusted if required.

This has been tested in HITL using AT_LEAST and MAX RTH types. There have been tests with linear descent enabled or disabled, and with varying start distances. All test results as expected. I will create some test videos shortly with other RTH types and with RTH Home Altitude set to 0.

Configurator

This change require configurator PR iNavFlight/inav-configurator#1727

Options to set up Linear Descent have been added to the Configurator.

If Linear Descent is enabled, and the RTH Home Altitude is less than 10m. A warning is shown around the RTH Home Altitude input box. People can still enter whatever values they want, including 0. If the RTH Home Altitude is not set, Linear Descent is not used in flight.

Fixes #8739

[Jetrell]

@MrD-RC The tests of this feature went well with a quad.
There was a slight headwind (6kph),

Setting were -
nav_rth_alt_mode = EXTRA
nav_rth_altitude = 3000 (30m on top of RTH activation altitude)
nav_rth_home_altitude = 1500 (15m)
nav_rth_linear_descent_start_distance = 100 (Linear descent starts 100m from home)

Linear.descent.radius.mp4

[MrD-RC]

HITL Tests

Firstly, I have to say that with the video capture software running in the background. The flight gets jumpy and the OSD data is a bit laggy. But, the numbers ultimately get hit. Plus, without the capture software, the flight is much more realistic.

Linear Descent inactive tests

Test 1 - AT_LEAST : Linear Descent OFF

I only ran one test with Linear Descent OFF, because the functionality is independent on the RTH type. So ON/OFF is the same for all.

https://youtu.be/w8zDatgi4Ms

Test 2 - AT_LEAST : Linear Descent ON - No home altitude set

If home altitude is set to 0. Linear Descent should be ignored, even if turned on. Again, I only ran this test once, as it is independent of RTH type.

https://youtu.be/Bl2JhCrzdxo

Linear Descent active tests

In all these tests, Linear Descent is turned ON. A RTH Home Altitude of 75 metres is set. And a Linear Descent start distance of 1500m.

Test 3 - AT_LEAST

https://youtu.be/-WqhLdVTdjw

Test 4 - MAX

https://youtu.be/PLMYaqD56tA

Test 5 - FIXED

https://youtu.be/rTbCV8SYiQ0

Test 6 - EXTRA

There are two parts to this test. Part one is how a normal RTH would be. In part 2, I tried seeing what would happen starting at a negative altitude.

https://youtu.be/jUVhHkTK-pM

Test 7 - CURRENT

https://youtu.be/cg8FCClzQ3U

[MrD-RC]

This will need to be 7.0 as it breaks compatibility. I originally set to the 6.1 milestone as there was no 7.0, and I expected 6.1 to just be renamed 7.0. I guess 6.1 will just be a pretty small release.

[MrD-RC]

Tested in flight. Worked as expected.