Popgen use-case

[jeromekelleher]

Further to discussion in #6

Here's a rough sketch of a three paragraph summary:

1. Who are the audience for popgen stats in sgkit, what do they do, and how do they do it? (ragbag mixture of tools)
2. Overview of sgkit's API methods
3. Concrete example based on the AG1000G notebook, giving timings, file sizes etc.

We may wish to update the notebook here and put it into this repo, and make a PDF version available as supplementary material.

Notes:

[tomwhite]

I've updated and cut down the original popgen repo, and have temporarily put it here: https://github.com/tomwhite/sgkit-publication-popgen. I'll move it to this repo later.