Added constants.py to API Reference
Added constants.py to API Reference

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