Development for two new features:

Development for two new features:
 * Some support for geometry information provided by the custom scenery
   project. Current support is for AI groundnets and runway use files only
   since this is a switch that involves a lot of data verification and
   updating, during the transistion the actual path where the data can be
   read from is user configurable. setting the property

   /sim/traffic-manager/use-custom-scenery-data to true

  will cause flightgear to read the ground networks from the scenery
  directory (--{fg-scenery}/Airports/[I]/[C]/[A]/[ICAO].groundnet.xml to be
  precise). Setting this property to false will retain the original
  behvior.
* For departing aircraft, runway takeoff calculations will be done on the
  basis of the performance database. For testing purposes, a performance
 estimate for a heavy jet has been added.