Namespace globals fuel

Fuel handling for the YASim FDM. Note that other FDMs (e.g. JSBSim)
handle fuel within the FDM itself.

Properties under /consumables/fuel/tank[n]:
+ level-lbs       - Current fuel load.  Can be set by user code.
+ selected        - boolean indicating tank selection.
+ capacity-gal_us - Tank capacity

Properties under /engines/engine[n]:
+ fuel-consumed-lbs - Output from the FDM, zeroed by this script
+ out-of-fuel       - boolean, set by this code.