Most subsystems are now created via the subsystem manager using the global
subsystem registrations.
The FGGlobals add_subsystem() and add_new_subsystem() methods have been removed
as the subsystem manager addition and creation function interface now exceeds
the functionality of these helper functions.
* Parking with point beyond parking so aircraft don't stop early
* Better approach routes (wait pattern)
* Extracting vector math from AIFlightplan
* More use of SGPositioned in ATC
* Relax runway exit route requirement to 80°
* Ensure parking is only reset if airport for AI aircraft has changed
* Heading Error signed and arrival lead distance
