FlightGear - Flight Simulator FGAddon

* Nasal/an2.nas (sbess_handler, left_tank, right_tank, total_fuel): delete.
* Systems/instrument-filters.xml: new; replace sbess_handler, left_tank,
right_tank from an2.nas.
* an2-set.xml (sim/systems/property-rule): new,
load Systems/instrument-filters.xml.
(nasal/fuel): delete.

* Nasal/fuel.nas: delete; settimer loop would compute fuel flows between tanks.
* Systems/fuel.xml: new, replace Nasal/fuel.xml.
* an2.xml: load it. Also specify AVGAS in the tanks, for density and energy density
computations.

* Instruments-3d/fuel/fuel_selector.xml: make the pick animations wrap around.
* Instruments-3d/switches/sw_fmeter.xml: use property-assign directly instead of the
nasal functions deleted from Nasal/an2.nas.

* Nasal/an2.nas (us): delete; settimer loop would simply convert knots
to km/h every frame for no reason.
* Instruments-3d/us-350/us-350.xml: use
instrumentation/airspeed-indicator/indicated-speed-kt directly
instead of an2/instrumentation/us-350/indicated-speed-kmh. Adjust interpolation
tables for the needle angles.