Added tests for Stumpff functions Closes #32.
Added tests for Stumpff functions
Closes #32.

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