Reference point HITL simulation

This simulation is working on the autopilot side (does not need any external data over telemetry), simulates the full behaviour of the AP in HOVER and NAV hotizontal (vertical) modes (assuming reference generation usage). Allows to fully simulate the flightplan with any extra modules/devices connected to the AP.
To turn it on, simply change in your airframe setup file the gps and ins subsystems to "sim_hitl":
<subsystem name="gps"           type="sim_hitl"/>
<subsystem name="ins"           type="sim_hitl"/>