Luca Grillotti

Hello! Now all the html files should have been updated in this PR (by using sphinx on the branch `new_doc`). If you want you can visualise the added documentation [there](https://lookatator.github.io/sferes2/qd.html)....

TODOs left: - [x] updating tests - [x] remove type aliases such as `PlayStepFnType` - [x] slight update of the documentation

> * is it possible to make the lambda function of the update_state a bit less verbose? Yup! Just did it in the last commit! > * what do you...