AHRS heading should be magnetic, not true

[westphae]

1. Is stratux GPS track magnetic or true?
2. AHRS "heading" (even though it's really track) should be magnetic.

Might have to implement World Magnetic Model to get variation right.

[westphae]

Verified: Stratux GPS track is true track; AHRS should report magnetic.
NMEA messages should report variation, but currently Stratux doesn't extract them, so may be possible to extract from NMEA; if not, can implement WMM.