Review and merge with `odf`(?)

Hi Robin (cc @agila5 and @rogerbeecham),

Great to see your progress on the `od` package!

Finally, I have made some progress on 'my' od package, which I called [`odf`](https://github.com/mtennekes/odf). As you will see below, `odf` doen't have as much features as `od`. I have put the emphasis on the data structure, for which I used a graph structure. However, I have a different approach than I had in mind during my visit in Leeds: now I'm following a much more light-weighted approach.

Could you take a look and see if it's useful for your package? I've rendered a [pdf from the vigette](https://github.com/mtennekes/odf/releases/download/0.1/vignette_2020_10_13.pdf) and also uploaded our [draft paper](https://github.com/mtennekes/odf/releases/download/0.1/Design_space_of_OD_visualizations.pdf). See also [the latest version of the 'halfline' doughnut visualization](http://www.mtennekes.nl/viz/commutingNL), for which I've partly used `odf`.

I have the feeling that we can merge our approaches quite well. Let me know what you think.

Start by reviewing the od package alongside the linked odf package, its rendered vignette, and the draft paper. Compare their data structures and features, then determine whether and how the approaches should be combined; done requires an agreed integration plan or merged implementation.

Written by the indexing model from the issue text.