[nps] 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.
[conf][nps] replace INITIAL_CONDITITONS with JSBSIM_INIT in all airframes
[conf] ground alt for versatile flight plans