Load XML file in FlightGear's native <PropertyList> format.
file will be located in the airport-scenery directories according to
ICAO and filename, i,e in Airports/I/C/A/ICAO.filename.xml
If the second, optional target parameter is set, then the properties
are loaded to this node in the global property tree. Otherwise they
are returned as a separate props.Node tree. Returns the data as a
props.Node on success or nil on error.
Usage: io.read_airport_properties(<icao>, <filename> [, <props.Node or property-path>]);
Examples:
var data = io.read_properties("KSFO", "rwyuse");