"engines die when any tank is empty" behavior. Otherwise, just
deselect the empty tank. This matches the "both" behavior many
lightplane fuel selectors have.
tanks created by the C++ code look empty, and were causing the fuel
code to detect out of fuel conditions. Since there is no way to tell
a "FDM" tank from a "C++" tank, I just filter them by capacity. Very
ugly hack, we need to fix the code to report only the tanks created
by the aircraft/FDM configuration.