Satellite database functions

[ryeng]

Add functions to access a system wide (and maybe also user specified) satellite database. Currently, every application needs to maintain it's own database of satellites, updating them every so often.

API functions:

• Load/access the entire database
• Look up a satellite by name/catalog number
• Add a satellite to the database (maybe, this requires write access)

It's not necessary to have update functions in the library, but if the user has write access, this could be a nice extra feature. Measures should be taken to avoid race conditions if a cron job is trying to update the database at the same time as an application reads/writes it.

If the same satellite occurs several times in one or more databases, which element set should be chosen? It's possible to identify the newest one. If it's possible to use the library functions to read different databases, applications could always maintain a separate database of overrides if necessary.

The TLE file format with the satellite name on a separate line (like the predict database) should be supported. Other, more efficient, formats could also be supported.