[nps] use magnetic field from airframe file if defined there

make it possible to disable the bypassing of the ahrs via define

sw/simulator/nps/nps_autopilot_rotorcraft.c

@@ -37,11 +37,15 @@
 struct NpsAutopilot autopilot;
 bool_t nps_bypass_ahrs;
 
+#ifndef NPS_BYPASS_AHRS
+#define NPS_BYPASS_AHRS TRUE
+#endif
+
 
 void nps_autopilot_init(enum NpsRadioControlType type_rc, int num_rc_script, char* rc_dev) {
 
   nps_radio_control_init(type_rc, num_rc_script, rc_dev);
-  nps_bypass_ahrs = TRUE;
+  nps_bypass_ahrs = NPS_BYPASS_AHRS;
 
   main_init();
 

sw/simulator/nps/nps_fdm_jsbsim.c

@@ -348,9 +348,16 @@ static void init_ltp(void) {
   fdm.ltp_g.y = 0.;
   fdm.ltp_g.z = 9.81;
 
+#ifdef AHRS_H_X
+#pragma message "Using magnetic field as defined in airframe file."
+  fdm.ltp_h.x = AHRS_H_X;
+  fdm.ltp_h.y = AHRS_H_Y;
+  fdm.ltp_h.z = AHRS_H_Z;
+#else
   fdm.ltp_h.x = 0.4912;
   fdm.ltp_h.y = 0.1225;
   fdm.ltp_h.z = 0.8624;
+#endif
 
 }