experimenting with UK ONSPD format

[missinglink]

This PR adds support for the ONS Postcode Directory (ONSPD) for the United Kingdom
https://geoportal.statistics.gov.uk/datasets/ons-postcode-directory-may-2020

I was interested to see what the coverage was like and found out:

• It's centroids only
• 2,643,729 rows in the file, each is a valid UK postcode
• Some of the rows have bunk coordinates

To use this importer, download the ONSPD_MAY_2020_UK.csv file from the link above and import it as such:

# import the whole file
cat ONSPD_MAY_2020_UK.csv | node bin/spatial.js import onspd

# import just the CSV header and postcodes starting with.. (for quick dev work)
cat ONSPD_MAY_2020_UK.csv | grep  -e 'ur01ind,oac01' -e '^"E8' | node bin/spatial.js import onspd

I had a quick play around with buffering the points to polygons and settled on a 0.001 radius, but this is obviously not an exact science.

cc/ @vicchi

[vicchi]

@missinglink Also worth looking at OS CodePoint Open as well; ONS PD is updated more frequently but CodePoint Open is more authoritative IMHO. Sadly, as long as 👑 ✉️ maintain their tight hold on the UK PAF (and I don't see that changing anytime soon) and with Open Addresses UK and Free The Postcode both now defunct, this is as good as it's going to get without resorting to proprietary licensed data, which will cost and not be usable in an open form anyway by definition.