AP_HAL_SITL: track simulated ADSB vehicles relative to simulation origin

removes dependence on ArduPilot AHRS library

Also removes vehicles based off vehicle simulated position rather than distance-from-origin, so you always have company

libraries/AP_HAL_SITL/SITL_Periph_State.cpp

@@ -294,6 +294,7 @@ SimMCast::SimMCast(const char *frame_str) :
 void SimMCast::update(const struct sitl_input &input)
 {
     multicast_read();
+    update_home();
     update_external_payload(input);
 
     auto *_sitl = AP::sitl();

libraries/AP_HAL_SITL/SITL_State.cpp

@@ -346,6 +346,7 @@ void SITL_State::_fdm_input_local(void)
     multicast_servo_update(input);
 
     // update the model
+    sitl_model->update_home();
     sitl_model->update_model(input);
 
     // get FDM output from the model