Nps for fixedwings

Make it possible to use NPS for fixedwings...

Also:
- configurable jsbsim model and initial conditions from flight plan
  - If NPS_JSBSIM_MODEL is not defined, it defaults to the AIRCRAFT_NAME (previous behavior).
  - If NPS_JSBSIM_INIT is defined, that ic file is used, otherwise uses flight plan location.
  - NPS_INITIAL_CONDITITONS is not used anymore.
- configurable joystick buttons/axes