Add support for cemetery tagged as grave_yard (#1175)

Fixes #1057

This PR adds `amenity=grave_yard` to the `landuse` layer.  A unification function was implemented which encodes all `class=grave_yard` as `class=cemetery` in the tiles, which adds these features for existing users of `class=cemetery` with no change.  The unification function can serve as a basis for any other tags that we might want to unify in the `landuse` layer.

Tile rendering for an `amenity=grave_yard`.  ([Location](https://www.openstreetmap.org/way/857383420))
![image](https://user-images.githubusercontent.com/3254090/129456504-187d307a-2a20-4b13-af0a-a2850503bb4f.png)

Tile rendering for a `landuse=cemetery`.  ([Location](https://www.openstreetmap.org/way/385779531))
![image](https://user-images.githubusercontent.com/3254090/129456654-494c0eb4-9785-467c-b1a0-44abb7f9d5e9.png)

layers/landuse/class.sql

@@ -0,0 +1,10 @@
+-- Unify class names that represent the same type of feature
+CREATE OR REPLACE FUNCTION landuse_unify(class text) RETURNS text LANGUAGE plpgsql
+AS
+$$
+BEGIN
+  RETURN CASE
+    WHEN class='grave_yard' THEN 'cemetery'
+    ELSE class END;
+END;
+$$;

layers/landuse/landuse.sql

@@ -49,14 +49,15 @@ AS
 $$
 SELECT osm_id,
        geometry,
-       COALESCE(
+       landuse_unify(
+         COALESCE(
                NULLIF(landuse, ''),
                NULLIF(amenity, ''),
                NULLIF(leisure, ''),
                NULLIF(tourism, ''),
                NULLIF(place, ''),
                NULLIF(waterway, '')
-           ) AS class
+           )) AS class
 FROM (
          -- etldoc: ne_50m_urban_areas_gen_z4 -> layer_landuse:z4
          SELECT osm_id,

layers/landuse/landuse.yaml

@@ -45,6 +45,7 @@ layer:
     geometry_field: geometry
     query: (SELECT geometry, class FROM layer_landuse(!bbox!, z(!scale_denominator!))) AS t
 schema:
+  - ./class.sql
   - ./landuse.sql
 datasources:
   - type: imposm3

layers/landuse/mapping.yaml

@@ -88,6 +88,7 @@ tables:
       - college
       - library
       - hospital
+      - grave_yard
       leisure:
       - stadium
       - pitch