Functions: exercises with scopes are not testable #269

There's some work to think thoroughly if we (ever) want to create exercises about scopes that can also be testable. I started it with 5825037 but it's far from complete. For the time being, I don't think it's "safe" to include these exercises.

Also: there will be conflicts to resolve with functions.ipynb, so I created a temporary copy of the notebook with the added exercises on scopes: functions_with_scopes.ipynb.