RehabCatchments about geospatialstroke HOT 1 CLOSED

@richardbeare That commit addresses all of those points. Respective notes on each:

1. Done. tmaptools is still used for geocoding because it remains the only simple approach that avoids an API key.
2. Done.
3. Done - check and confirm, but I think I've got it right this time?
4. Interesting this one - that simply required calculating initial postcode distances to the polygon centroids. This gives quite different results to (former version of) just st_distance(POLYGON, POINT), which defaults to the shortest distance, and so ultimately includes more polygons. (These centroid calculations, both in python and R, are of course strictly invalid because you can't calculate the centroid of a lon-lat polygon, but I've noted that in the text, along with a claim that any inaccuracy so arising can be ignored for our purposes.)

from geospatialstroke.