Simplify some of the OSM->OMT field value mappings using declarative syntax.
This approach is not for all cases, but in many it removes
the need of storing the same field in both the .yaml and .sql files.
TODO: support more complex AND/OR cases
The `camp_site` subclass is assigned to the `campsite` class by an earlier statement"
```sql
WHEN subclass IN ('camp_site','caravan_site') THEN 'campsite'
```
* rail stations
* Create 'railway' POI class with 'station' and 'halt' subclasses
* Introduce subway stops as POI
* Revert unwanted changes in .env and openmaptiles.yaml
* Remove halt key from documentation
* Distinguish aeroway and railway station, add tram_stop