ekf2: separate mag and mag heading control logic (#21212)

- split mag_3d into new standalone mag fusion and mag fusion allowed to update all states (full mag_3d)
 - new dedicated control logic for mag/mag_3d fusion and standalone mag heading fusion
 - if WMM available use for mag_I and mag_B init
 - mag states reset if external yaw reset (yaw estimator, GPS yaw, etc)
 - mag reset if declination changed (eliminate _mag_yaw_reset_req)
 - mag fusion (but not mag_hdg or mag_3d) can be active during gps_yaw or ev_yaw (if yaw aligned north)

Co-authored-by: bresch <brescianimathieu@gmail.com>

src/modules/ekf2/EKF/ekf.cpp

@@ -45,9 +45,12 @@
 
 bool Ekf::init(uint64_t timestamp)
 {
-	bool ret = initialise_interface(timestamp);
-	reset();
-	return ret;
+	if (!_initialised) {
+		_initialised = initialise_interface(timestamp);
+		reset();
+	}
+
+	return _initialised;
 }
 
 void Ekf::reset()