In preparation for caching the groundnet in the NavCache, make taxi-nodes and parkings inherit from FGPositioned. As part of this, make them heap (as opposed to value) classes, disable their copy-constructors, remove many mutating operations, and give them real constructors.
Cache the parsed navigation and airport data in a binary file to reduce
startup times and memory consumption (since only referenced FGPositioned
elements are held in memory).
Data will be reimported when the mod-time of any input file is changed.
If a global file is changed (nav.dat, awy.dat, apt.dat, etc), the cache
will be completely rebuilt, which takes approximately 30 seconds on
moderate hardware. (Future work may reduce this).
Separate other init functions from position init, since they are unrelated, and made fg_init.cxx very complex. Next step will be brining some sanity to the position init logic :)
