typo; the SimGear "pass subproperty as first argument" shortcut
doesn't work in Nasal. This checkin is blind, but should have a high
probability of correctness.
"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.
