Add altitude-hold PID method

[antholuo]

Description

Patch to add PID modes for holding altitude and hovering in place.

This operates on the principle that when a switch is toggled or command otherwise set, the drone will hover in at the altitude / state that it is in when the toggle occurs.

Altitude Hold

• On Toggle, the current altitude is saved and the drone will attempt to maintain that altitude.
• Throttle stick is de-activated and there is currently no way to adjust the height of the drone in this state. Looking for ideas.
• Uses current roll/pitch corrections to maintain position.
• Will maintain yawRate of 0, which should be equal to minimal change in yaw.

Position Hold

• On toggle, current position is saved and drone will attempt to maintain current latitude, longitude, altitude, and heading.
• Do we want to re-map controls inputs to move the position around? Perhaps just with slew control so that fine adjustments in positioning are easier?
• Currently does not interface with any gps data, subject to imu drift.
• Uses same yaw, altitude PID as altitude hold.
• Uses 2 new PID's for latitude and longitude. Each PID is driven off a difference in location from current and target, and will compensate the 2 sets of motors in opposite directions to return to the position.

Include the Asana link, or relevant issue.
Best thing I can do is link to the discord, was a quick patch.

https://discord.com/channels/776618956638388305/895818568065757184/969037598871937054

Testing

Safety builds. Will need to be tested on drone.

Builds of existing PID tuned code.

Documentation

Merge Checklist:

• The changes have been well commented, particularly in hard-to-understand areas.
• The code has been tested on hardware, either by me or somone else.
• Comprehensive unit tests have been made for this change
• Corresponding changes to documentation have been created and links to this documentation are provided within the pull request.
• The changes generate no new warnings and compile and run; A screenshot of a successful compile message is attached to the bottom of this PR.

Screenshot: