Added
- Integration of JPL ephemeris by interfacing Brandon Rhodes' [jplephem](https://github.com/brandon-rhodes/python-jplephem) python library
- First try at RK4 numerical propagator
- Listeners for events computation (AOS, LOS, umbra, etc.)
- CCSDS Orbit Data Message reading and writing
- Multi TLE parser (18)
- frames now declare a central body, with some characteristics (20)
Changed
- Spherical parameters orders (now r, theta, phi)
- Propagators are now instances instead of classes
- ``solarsystem`` module
Fixed
- Correction of velocity computation when switching from cartesian to spherical
- COSPAR ID parsing in TLE