When a position is modified for an in-cache FGPositioned, we need to update the runtime information too, or the Octree code may (rightly) complain that it's seeing inconsistent data. Also make the Octree check an exception throw, and verbose, so this is easier to detect in the future.
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).
