Refactor

Started refactoring of poliastro.

- [ ] `Body`, pretty printing.
- [ ] `State` objects, conversion between coordinates, test Vallado 25 and 26
- [ ] Propagation, start with Kepler orbits, reimplement algorithm from Vallado, test Vallado 24, get rid of corresponding Fortran
- [ ] `Maneuver` objects, Hohmann and bielliptic, test Vallado 61 and 62
- [ ] Ephemerides, use jplephem (see #4), test Vallado 55 and others
- [ ] Lambert problem
- [ ] Targetting