Spending more time on `groupby()`

[tomvannuenen]

Some of the participants asked if there could be more time spent on groupby(), as they need it for their research. I feel it's important to spend at least 20' on this section and challenge. Not sure if I have an immediate idea for how to do this -- another challenge after #10, giving more examples of groupby() functionalities or scenarios, introducing it earlier (e.g. before merging).