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.
Add a bool flag isRoute to flight-plan, and use it to model
routes (which contain VIAs) from plans/legs, which do not.
Default isRoute to false, so most users only see expanded
Vias, and hence something that can be flown directly.
Overhaul how transitions are stored in FlightPlan XML, and how
they’re exposed to Nasal. Simplify the Nasal access by making
‘sid_trans’ and ‘star_trans’ writeable.
Extend the unit-tests a lot to cover this, both from C++ and also
from Nasal
As part of this, overhaul the ownership of FlightPlan delegate
factories, to make it safer (use ref-counting of the factories,
and allow the factory to customise delegate clean-up behaviour)
One test so far, attempting to reproduce a bug I see with the 737.
Note these tests are skipped unless you have procedures available
and set a magic env var to find them.
