Improve re-running the merger

[stefanpernpaintner]

Currently when re-running the merger one has to basically throw away the db and has to recreate it. A more convenient way is to persist merged stations isolated from the imported data and only delete the according table.
Further ideas:

• this makes the flag in the stations table redundant that shows the merge status hence this can also be deleted.
• The info of MergedStationSource can be stored in the new merged_stations table and MergedStationSource entity and table can be removed

[LarsHaferkampEON]

Current workaround (includes adding is_merged flag to address and charging table).
After deleting the entries the merger can run again

  delete from echarm_merged_station_source ;
   delete from echarm_charging a where a.is_merged=true;
   delete from echarm_address a where a.is_merged=true;
   delete from echarm_stations es where es.is_merged=true ;
   update echarm_stations SET merge_status = null;

[LarsHaferkampEON]

For now implemented a new option (delete_data), which performs the actions above, will create a PR