Implement parser for Code-Point Open postcode data.

  Todo:
  - Evaluate "convert northings/eastings to lat_lon" solution's accuracy.

  - Evaluate "compute the centre of a district" solution's accuracy.
    We calculate it as the means of each lat and lon.

  - Find a better way to store parsed data.
    The 1.7 million files generated will currently claim about 10GB on a filesystem using 4K-blocks.

  - Implement as convenience capability:"Download and unzip the latest archive and unzip it to ./data".