Refactor

Started refactoring of poliastro.

- [x] `Body`, pretty printing.
- [x] `State` objects, conversion between coordinates, test Vallado 25 and 26
- [x] Propagation, test Vallado 24
- [ ] Reimplement algorithm from Vallado, benchmark, get rid of corresponding Fortran
- [ ] `Maneuver` objects, Hohmann and bielliptic, test Vallado 61 and 62