Read groundnet node information from the scenery, and (optionally) write it to a...

Read groundnet node information from the scenery, and (optionally) write it to a cache file. Note that this mechanism should be extended, so that the cache files will be updated whenever the terrain files are changing. The current patch also uncovers a bug, in the sense that the groundnetwork destructor is never called. Presumably this is due to the way airports are allocated in flightgear. Because the ground networks are never saved, I am periodically saving the cache files; but this behavior should probably be changed later on.

The current patch incorporates work contributed by Adrian Musceac.