Clean up predict_orbit_t

[ryeng]

Some values seem to be duplicated, or maybe even triplicated by pointers to structs/character strings with the same information.

A clean separation between predict_tle_t and predict_orbit_t would help a lot. Currently, predict_tle_t is only used deep down in the library. It makes sense to parse a TLE string into predict_tle_t and then give the predict_tle_t as parameter to predict_create_orbit() instead of the string array it currently takes.

[ryeng]

See also issue #22.

[bjorgan]

Time-dependent, calculated orbit number will be called "revolution" (issue #30). The orbit number at epoch in the TLE-field should be called something like "revolution_at_epoch" in order to not cause confusion and to highlight the connection between the two.