Sub: Improvements to depth hold, new depth hold tests

[Williangalvani]

Before:

Note that it was moving upwards with the throttle down.

After:

Another noticeable change is that when climbing/diving Alt and DAlt do not match anymore. That is because the old code just applied thrust and relaxed the controller (basically doing DAlt = Alt).

This still needs real life testing to make sure that response is not too sluggish
I tested on a BlueROV2 and it feels good.

Log:
00000001.zip

[jaxxzer]

Open a separate pr to add position_control.get_throttle_output, and use that in place of motors.get_throttle_input please.

[rmackay9]

I think it is best to keep passing the throttle through the attitude controller to keep consistent with Copter.

[lthall]

This looks good!

[Williangalvani]

Thanks for the reviews. I addressed Randy's review.