pipeline automation using the doit library for python

A lesson on using doit for automating data analysis pipelines. The aim would be to eventually cover the material which we currently have for make.

This was meant to be a 10 minute lesson for Instructor round 8.5, but it was far too long so I split it into three lessons of roughly 10 minutes each.

I'm not sure whether the comments on this pull request are the best place to discuss the relative merits of teaching doit vs. make (possibly that ought to be an issue rather than a PR). Just for some context though, here is a short summary of my pitch:

### Advantages:

- doit is much more verbose than make, which I think makes the files more readable if you know neither make nor doit
- doit can be combined with argparse to make a pipeline with a nice usage page and sensible, verbose parameters. I think this is a big plus for maintainability
- A build tool written in python might be less cognitive overload if learners are covering python in the same lesson

### Disadvantages:

- Introduces an additional dependency to install
- Very probably many fewer SWC instructors are familiar with doit than make