Add 'natural=volcanal' into mountain_peak layer

1. Add new field "class" into mountain_peak layer,
   which is original value of "natural" tag
2. Reformat layer.sql

layers/mountain_peak/mountain_peak.yaml

@@ -8,13 +8,19 @@ layer:
     name: The OSM [`name`](http://wiki.openstreetmap.org/wiki/Key:name) value of the peak.
     name_en: English name `name:en` if available, otherwise `name`.
     name_de: German name `name:de` if available, otherwise `name` or `name:en`.
+    class: 
+      description: |
+        Use the **class** to differentiate between mountain peak and volcano.
+      values:
+      - peak
+      - volcano
     ele: Elevation (`ele`) in meters.
     ele_ft: Elevation (`ele`) in feets.
     rank: Rank of the peak within one tile (starting at 1 that is the most important peak).
   datasource:
     geometry_field: geometry
     srid: 900913
-    query: (SELECT osm_id, geometry, name, name_en, name_de, {name_languages}, ele, ele_ft, rank FROM layer_mountain_peak(!bbox!, z(!scale_denominator!), !pixel_width!)) AS t
+    query: (SELECT osm_id, geometry, name, name_en, name_de, {name_languages}, class, ele, ele_ft, rank FROM layer_mountain_peak(!bbox!, z(!scale_denominator!), !pixel_width!)) AS t
 schema:
   - ./update_peak_point.sql
   - ./layer.sql