Use libpredict for orbit calculations

[ryeng]

The orbit calculation algorithms have been moved into libpredict (https://github.com/la1k/libpredict). Flyby should use the library instead of its own algorithms.

Until libpredict adds support for a TLE database, predict should use its own. Even after libpredict adds support, it might be useful to have a separate flyby TLE file.

[bjorgan]

I've split this issue into issue #5, #6, #7, #8, #9, #10.

Each function should be rewritten in a way that it uses no global variables related to orbit parameters or the satellite database.

Close this issue when the above issues have been resolved, and all global variables and functions related to orbit calculations can safely be removed from flyby.c.

[bjorgan]

Fixed in a330e0c