Previously the GenericFuelPublisher would barf if tank contents
weren't defined properly with numbers.
Also the XML and .ac files were inconsistent in their use of PANDown
vs PanDown.
Both spotted by Mark JOSEPH (ePilot). Now fixed.
Add Multikey support for the FG1000 (:GF and :GP) to improve useability
by allowing direct entry of strings. This is massively easier than
fiddling with the control knobs.
Also correct interface variable name "frequency" to "period", which is
more accurate.