Remove various hacks and make magvar work like a normal subsystem, as part of the environment manager. Fix the remaining users of the globals->get_mag accessor, and hence kill off the global pointer.
Compute gravitational acceleration based on the formula of
Somigliana corrected for altitude and provide the value
in property /environment/gravitational-acceleration-mps2
make options
--wind=nnn@mm
--visibility
--visibility-miles
--ceiling
--turbulence
work again. These options now write properties in the
/environment/config/presets
branch. The values will be applied in the property-rules
in FGDATA/Environment/interpolator.xml
