Converge IMU faster at higher groundspeed during GPS Rescue (#12738)

* dynamic dcm_kp depending on groundspeed vs velocity to home

* reduce max pitch angle default to 40

* Only 10% of set velocity to home is enough, angle to 45

to reduce chance of sanity check against headwind

* fixes, thanks @KarateBrot

* Feedback from Ledvinap, ensure full pitch and yaw authority in descend

* pitch limit max of 1/3 and dcm_kp gain to 6x max

* apply IMU kP factor to yaw only via ez_ef

* More from @Ledvinap, constrain pitch in descent also

* Fix the velocity error gain, refactoring thanks to Ledvinap

* thanks, Mark!

* fixes, must be fabsf

* make IMU gain twice as sensitive to groundspeed error

* useCOG as float

* Fix unit test

* ensure cogYawGain is zero, except during gps rescue

* update comment after suggestion from ledvinap

* keep roll angle limit consistently at 50% of pitch limit when active

* changes to comments, thanks petr

src/test/unit/flight_imu_unittest.cc

@@ -256,6 +256,7 @@ extern "C" {
     float baroUpsampleAltitude()  { return 0.0f; }
     float pt2FilterGain(float, float)  { return 0.0f; }
     float getBaroAltitude(void) { return 3000.0f; }
+    float gpsRescueGetImuYawCogGain(void) { return 1.0f; }
 
     void pt2FilterInit(pt2Filter_t *baroDerivativeLpf, float) {
         UNUSED(baroDerivativeLpf);