Goals for exercises

There should be enough exercises that students can get help on a number of exercises, and then still have enough to complete a number of exercises independently. If students don't get to the point of writing simple programs independently, they will not get far in programming.

This can easily clutter up the notebooks. I have several thoughts about managing this:
- Make the exercises collapsible. When the notebook loads, exercises are hidden. When a student is ready to do the exercises, they click a _show exercises_ button, and they can see the exercises.
- Include 2-3 exercises in each teaching notebook, and then link to a separate notebook with more exercises.