Change local reverse geocoder recommendation

[dgraf-gh]

Since I am working on #180 I discovered that the current Photon recommendation is not the best option in terms of data quality.

There is also the nominatim available as a docker container which can also import a dataset on startup.

The options are now:

• replace photon with nominatim in the default docker-compose file, but keeping the support for Photon
• keep Photon as it is and add documentation to switch it out to nominatim, also adding a new fixed nominatim geocoder option

I am hesitant since every user has to upgrade its compose-file to use the local nominatim instance and the configuration is slightly more difficult. At the moment the user has to change COUNTRY_CODE: de to its main country code. With nominatim, your first have:

• go to http://download.geofabrik.de/
• choose the desired dataset navigating down the hirarchy. Like http://download.geofabrik.de/europe/germany.html
• then copy the link http://download.geofabrik.de/europe/germany-latest.osm.pbf and replace the default config in the compose file with

      PBF_URL: http://download.geofabrik.de/europe/germany-latest.osm.pbf
      REPLICATION_URL: http://download.geofabrik.de/europe/germany-updates/