AP_ExternalAHRS_AdvancedNavigation

[AN-DanielCook]

This PR Adds External AHRS support to Advanced Navigation Devices using the Advanced Navigation Packet Protocol.

when EAHRS_TYPE = 3 (AdNav) the new module will load and run. A SITL Emulator of a ANPP Device has also been added for easier testing in the absence of hardware.

[tridge]

@AN-DanielCook great to see this driver, thanks. As @peterbarker says, this needs some substantial re-work. That re-work will be made much easier because you've done a simulator, which is great! If you need help with the re-work please reach out (though would need to be after the conference)

[AN-DanielCook]

Sorry I've only recently had some time to come back and apply changes to this. If you are happy with the structural changes in the driver i will apply them to the simulator also.

[tridge]

@AN-DanielCook @peterbarker I've rebased, fixing conflicts and fixed a few other bugs. sim_vehicle.py now works like this: sim_vehicle.py -D -G --console --map -A --serial2=sim:AdNav Some key changes:

• lat/lon was ordered opposite in driver and simulator. I've changed the simulator, but it could be the driver that is wrong
• the simulator was sending 50Hz GPS, which is much too high. If the real device sends 50Hz GPS we will need to downsample in the driver
• time handling was broken

[tridge]

@AN-DanielCook I pushed a few fixes to this PR today

[tridge]

@AN-DanielCook I've rebased this PR and fixed the conflict