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.
Ensure changes from Nasal are picked up by delegates (such as GPS) when
they are made. Add a route-manager test which sets and exits a hold
using Nasal.
As part of this, update the test API to make it easy to run Nasal from
a test.
The GPS/RNAV system can now fly holds, both left and right-handed.
The correct entry is selected, and hold count can be selected.
Also there's lots of lovely test cases.
