Return GeoJSON centroids for each postcode in Luxembourg

- Downloads the latest geojson from UDATA_ADDRESSES
- Average the position of all postcodes
- Spit out geojson

Run this command to install the requirements:

`python3 -m pip install -r requirements.txt`

and then:

`python3 postcode-centroid.py > centroids.geojson`

A sample centroids.geojson.xz (from 2016-07-11) is included.

There are 63 postcodes in Luxembourg that contain only one
address. Some of these are for residential addresses. You
might want to consider merging these points with the nearest
neighbour if you need to anonymise the output data of
your project.