Add park layer

layers/park/class.sql

@@ -0,0 +1,7 @@
+CREATE OR REPLACE FUNCTION park_class(landuse TEXT, leisure TEXT, boundary TEXT) RETURNS TEXT AS $$
+    SELECT CASE
+        WHEN leisure = 'nature_reserve' OR boundary='national_park' THEN 'national_park'
+        WHEN landuse IN ('recreation_ground', 'village_green') or leisure = 'park' THEN 'public_park'
+         ELSE NULL
+     END;
+$$ LANGUAGE SQL IMMUTABLE;