Improve user guide
Improve user guide

* Add text representation of orbits
* Update some old commands
* Add more explicit imports

newlawrence/poliastro,Juanlu001/poliastro,Juanlu001/poliastro,Juanlu001/poliastro,anhiga/poliastro,newlawrence/poliastro,anhiga/poliastro,anhiga/poliastro,newlawrence/poliastro,poliastro/poliastro