Added
-----

- OrbitInfo for hyperbolic orbits
- Kepler and J2 analytical propagators
- Beta angle computation
- Constellation and LEO builder
- Local Time at Ascending Node computation module
- Real adaptive step size for KeplerNum propagator
- Runge-Kutta-Fehlberg method for KeplerNum
- Lambert's problem solver
- ``ccsds`` now keeps track of "USER_DEFINED" fields

Modified
--------

- ZeroDoppler listeners renamed as RadialVelocity
- The Kepler numerical propagator (now renamed as KeplerNum) use Ephem
  objects for interpolation
- NonePropagator is not used anymore when ``orb.propagator = None`` and
  has to be explicitly passed
- Hyperbolic orbits are much better now that their computations are
  simply done right
- ``find_event`` uses ``events_iterator``, and Listeners are cleaned of
  residual states before each iterator creation
- ``ccsds`` as a single subpackage and homogeneous internal functions