Handling of hierarchical postcodes

[lonvia]

Some countries like the UK or the Netherlands have hierarchical postcodes where a part of the postcode can be left out to designate a larger area. In OSM both the long and short form may be mapped. And a user may search for either of them.

Nominatim should be able to match hierarchical postcodes against any longer and shorter form. A couple of thoughts what needs to be done for this:

• Parsing of country-specific postcode formats is required for import and search.
• On import/update, short form centroids need to be derived from longer forms for the postcode table. They should be marked, so that they are not used for the calculated postcode in places.
• On search, postcodes need to be looked up from longest to shortest form.

[dracos]

I would really like this because e.g. a search for https://nominatim.openstreetmap.org/search.php?q=Main+Road%2C+Bromley returns two roads, one in TN16, one in BR5 (15 results for the first, then 4 for the second), but https://nominatim.openstreetmap.org/search.php?q=Main+Road%2C+Bromley%2C+BR5 returns no results, and https://nominatim.openstreetmap.org/search.php?q=Main+Road%2C+BR5 returns results in Birmingham (a bus stop called BR5, no Main Road, though) or Fiji.