Suggestion: Put solutions in .py files

[sciunto]

The idea comes from https://github.com/amueller/scipy-2018-sklearn

Instead of duplicating the notebooks, they use the %load instruction to load py files stored in a solution directory. I think it's brilliant. I moved to this way for my notebooks; it will be much easier to maintain the code, and it makes the repo lighter.

For the tutorial, the lecturer can either provide the solutions or not, can type the solution or load it...

I have to admit that moving to this solution takes some time... :(

[jni]

I like this idea in many cases but it doesn't capture the spontaneity of the tutorial — often students will ask questions that deviate from the original lesson, and it's good to have those recorded. That's harder to do with the .py files vs depositing the notebook somewhere after the tutorial.