Add an option to include magnetometer

[AdrianEddy]

Magnetometer readings are useful to calculate better orientation, so it would be nice to have an setting to enable logging magnetometer too

[VladimirP1]

The code is actually logging quantized and compressed using rans encoding angular acceleration values. AHRS is running onboard. This means this would also require mag support in the built-in AHRS and mag calibration support.

Will look into this after implemention accel calibration (currently uncalibrated), it should be pretty similar to mag calibration.