FlightGear - Flight Simulator FGAddon

On the real aircraft, tanks were refuelled individually and measured using a
combination of internal and external dipsticks, so loading exact quantities and
having an exact balance between tanks is not realistic.

This commit introduces a configurable error percentage when refuelling, which
provides more realistic refuelling tolerances. It also leads to more realistic
fuel management during a flight because opposite tanks need to be managed into a
state of balance.