Implement custom Halley method
Implement custom Halley method

See https://github.com/scipy/scipy/issues/5922 for a potential bug
in `scipy.optimize.newton` and a brief analysis of convergence.
For the results that led me here, see
https://github.com/poliastro/poliastro/wiki/Izzo-algorithm-for-Lambert's-problem

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