[nps] correction between geocentric and geodetic ecef pos

This correction is required since jsbsim ecef is based on geocentric
model and pprz on geodetic model. This result is that lla pos computed
from ecef is wrong. It is not an issue if you use global coordinates or
if you reset the ecef ref, but it fails if only the reset is only on
altitude (NavSetAltitudeRefHere).
The correction is only computed when using flight plan init (no
JSBSIM_INIT defined in the airframe file), since the
original geodetic lla pos is required and jsbsim init xml files are
directly using geocentric pos.