Make checkable exercises

[wfmackey]

At the end of each chapter, we should provide a task that people could go away and do.

The task would be clearly defined and small, eg:

• read in this csv
• rename and remove these variables
• find the average (whatever) of (whatever group)
• write the result to a csv

Then could submit code to a reviewer to provide feedback

[MattCowgill]

agreed