Adding sitemaps for addresses

This PR adds

1. Sitemaps containing all adresses of our wohnungsregister layers. It contains links as [1]. These xml files are gzipped (each ~500KB, in total ~20MB)
1. Extends the create_sitemap/update script to create these files. This shouldn't be necessary often.

[1] http://map.geo.admin.ch/?ch.bfs.gebaeude_wohnungs_register=1761259_0&X=199101.00&Y=626766.00&zoom=9