ekf2: Fix EKF preflight check fail false positives

Uses a relaxed yaw innovation check threshold for fixed wing vehicle that can recover from larger yaw errors after takeoff.
Replaces the peak hold with decay filtering applied to the magnetic yaw innovations with a conventional lowpass filter. This prevents mag heading innovation transients caused by preflight handling from failing the check, but catches persistent errors. Sign swapping of innovations due to angle wrapping is not a problem due to the way that innovations are calculated inside the EKF, so a simple LPF sufficient.

src/modules/ekf2/ekf2_main.cpp

@@ -1397,18 +1397,20 @@ void Ekf2::run()
 
 					float yaw_test_limit;
 
-					if (doing_ne_aiding) {
-						// use a smaller tolerance when doing NE inertial frame aiding
+					if (doing_ne_aiding && vehicle_status.is_rotary_wing) {
+						// use a smaller tolerance when doing NE inertial frame aiding as a rotary wing
+						// vehicle which cannot use GPS course to realign heading in flight
 						yaw_test_limit = _nav_yaw_innov_test_lim;
 
 					} else {
-						// use a larger tolerance when not doing NE inertial frame aiding
+						// use a larger tolerance when not doing NE inertial frame aiding or
+						// if a fixed wing vehicle which can realign heading using GPS course
 						yaw_test_limit = _yaw_innov_test_lim;
 					}
 
-					// filter the yaw innovations using a decaying envelope filter to prevent innovation sign changes due to angle wrapping allowinging large innvoations to pass checks after filtering.
-					_yaw_innov_magnitude_lpf = fmaxf(beta * _yaw_innov_magnitude_lpf,
-									 fminf(fabsf(innovations.heading_innov), 2.0f * yaw_test_limit));
+					// filter the yaw innovations
+					_yaw_innov_magnitude_lpf = beta * _yaw_innov_magnitude_lpf + alpha * constrain(innovations.heading_innov,
+								   -2.0f * yaw_test_limit, 2.0f * yaw_test_limit);
 
 					_hgt_innov_lpf = beta * _hgt_innov_lpf + alpha * constrain(innovations.vel_pos_innov[5], -_hgt_innov_spike_lim,
 							 _hgt_innov_spike_lim);