Parking position should also be read from apt.dat

Problem:

Flightgear does not read parking positions from apt.dat.gz That is sad because it contains much more than what we have in ICAO.groundnet.xml files.

http://sourceforge.net/p/flightgear/codetickets/1169/

Workaround:

scripts which generate ICAO.groundnet.xml files from apt.dat entries. see: http://forum.flightgear.org/viewtopic.php?f=5&t=25779 http://forum.flightgear.org/viewtopic.php?f=5&t=21544#p196239

many groundnets with parking positions were converted from apt.dat and uplaoded to the terrasync servers.

possible Fix(?):

a function like parseParkPos(simgear::strutils::split(line)); in Airports/apt_loader.cxx