Rail stations 2 (#255)

* 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

layers/poi/poi.yaml

@@ -20,6 +20,8 @@ layer:
           [`amenity`](http://wiki.openstreetmap.org/wiki/Key:amenity),
           [`leisure`](http://wiki.openstreetmap.org/wiki/Key:leisure),
           [`landuse`](http://wiki.openstreetmap.org/wiki/Key:landuse),
+          [`railway`](http://wiki.openstreetmap.org/wiki/Key:railway),
+          [`station`](http://wiki.openstreetmap.org/wiki/Key:station),
           [`sport`](http://wiki.openstreetmap.org/wiki/Key:sport),
           [`tourism`](http://wiki.openstreetmap.org/wiki/Key:tourism)
           or [`shop`](http://wiki.openstreetmap.org/wiki/Key:shop)
@@ -34,6 +36,7 @@ layer:
     srid: 900913
     query: (SELECT geometry, name, name_en, name_de, class, subclass, rank FROM layer_poi(!bbox!, z(!scale_denominator!), !pixel_width!)) AS t
 schema:
+  - ./poi_update.sql
   - ./poi_polygon_update.sql
   - ./class.sql
   - ./layer.sql