AP_NavEKF3: set a min yaw accuracy from GPS of 5 degrees

GPS modules tend to be rather optimistic about their yaw accuracy. By
setting a min or 5 degrees we prevent the user constantly getting
warnings about yaw innovations

libraries/AP_NavEKF3/AP_NavEKF3_Measurements.cpp

@@ -622,6 +622,12 @@ void NavEKF3_core::readGpsData()
             // if the GPS has yaw data then input that as well
             float yaw_deg, yaw_accuracy_deg;
             if (AP::gps().gps_yaw_deg(yaw_deg, yaw_accuracy_deg)) {
+                // GPS modules are rather too optimistic about their
+                // accuracy. Set to min of 5 degrees here to prevent
+                // the user constantly receiving warnings about high
+                // normalised yaw innovations
+                const float min_yaw_accuracy_deg = 5.0f;
+                yaw_accuracy_deg = MAX(yaw_accuracy_deg, min_yaw_accuracy_deg);
                 writeEulerYawAngle(radians(yaw_deg), radians(yaw_accuracy_deg), gpsDataNew.time_ms, 2);
             }