simplify brexit using topojson

[ljwolf]

This needs to swap out for `../data/simple_brexit.shp' in the datasets.

Also, stupidly, I think we should call this just "brexit.shp" and remove the kinds of "clean" or "simple" from our processed datasets.

[darribas]

OK @ljwolf I've given it a pass. A few changes:

• PR now has Ch. 6 and 7 running off the new, simplified data
• Cleaning notebook has a bit more verbose and links to the datasets
• I've swapped simple_brexit for local_authority_districts which is what it represents (Brexit data is read separately)
• I've saved the simplified geoms as a GeoJSON, not a shapefile so they're more easily linkable over the web

I think this is good to merge afaik