When leg spans more than a threshold (currently 0.5 degrees) of longitude, interpolate the actual path flown by the GPS/RM, which is a true great-circle. Previous we rendered a Rhumb line which does not agree at all. (Especially noticeable in the MapWidget and NavDisplay code, both of which use RoutePath to tesselate the route before rendering)
Convert the route-manager to use a flight-plan internally, and expose
flightplan, leg and procedure data to Nasal. Move the Level-D parser
into its own file.
