[j-w9]

"Option to use GSF and GPS yaw as truth" from #81.

New attitude sources: EKF3 roll and pitch with yaw from the GSF yaw estimator (XKY0.YC), or from any GPS instance that logs yaw (GPS.Yaw, which is logged as 0 when there is none). Neither uses the compass.

• GSF samples are used when YCS is under the EKF's own reset threshold, GSF_YAW_ACCURACY_THRESHOLD_DEG (15 deg). YCS is logged as sqrt(degrees(variance)), so the threshold is converted to match (about 1.98).
• The new attitude is built at the EKF quaternion times with interpolated yaw. Samples with more than 1 s between valid yaw samples are skipped.
• These sources only cover part of the flight, so selecting one limits the analysis time to that range rather than changing the fit code.
• The sources only appear when the log has valid data, and the default selection is unchanged. Default fits are identical to main on four logs.

Tested on a SITL flight with an NMEA GPS emitting HDT (GPS1_TYPE 5, SIM_GPS1_TYPE 5, SIM_GPS1_HDG 1), flying a square and full yaw turns. Mean yaw difference from ATT: GSF 5.6 deg, GPS 2.2 deg. Offsets with EKF / GSF / GPS yaw: (3.0, 16.9, -17.7) / (4.2, 14.9, -17.6) / (3.1, 14.3, -17.5).