Implement a persistent cache for navigation data.

Implement a persistent cache for navigation data.

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).