New UK data sources

@mem48 pointed me towards some datasets for the UK. https://uprn.uk/ could help towards estimating number of housing units per building for weighting trips to/from homes. Relatedly I stumbled on https://longair.net/blog/2021/08/23/open-data-gb-postcode-unit-boundaries/.

There's also a geotiff with elevation data based on https://www.ordnancesurvey.co.uk/business-government/products/terrain-50. It should hopefully be very easy to add to https://github.com/eldang/elevation_lookups.

I'll try out both when I have time, but writing down the links before I forget