Reset system constants between examples

[ixjlyons]

Per student feedback, it'd be good to get into the habit of re-establishing system constants each time a system is used in case they modify them during class.

In general, this brings up an interesting point about both classwork and homework notebooks in that some cells are sort of meant to be read-only and they should insert their own cells (that they can delete) if they want to change things. You can actually add "editable": false to the cell metadata but there's no indication of this in the UI so it's a bit weird.