Add more description to the OO intro

[PythonCHB]

Currently all the description about what to do is in comments in teh code. Which has mostly worked, but it would be helpful to have a bit more description of the goal of the class.

And make it clear that they need to add tests to fully test their code.

It turns out that if we give them some tests, they think they are done if they all pass. So we need to either give them complete tests, or make it very clear that they are NOT complete, and that theyneed to add more.