Add class=pond to the water layer (#1456)

This PR proposes to add a new class, "pond" to the water layer, which allows for a style to de-emphasize minor bodies of water (tagged `water=pond,basin,wastewater`) when compared to more significant objects tagged reservoir/lake.  This PR retains the default behavior of classifying a water body as a "lake" if more specific tagging is not added.

layers/water/mapping.yaml

@@ -104,4 +104,7 @@ tables:
       - dock
       water:
       - river
+      - pond
+      - basin
+      - wastewater
     type: polygon

layers/water/water.yaml

@@ -27,6 +27,7 @@ layer:
           All water polygons from [OpenStreetMapData](http://osmdata.openstreetmap.de/) have the class `ocean`.
           Water bodies with the [`water=river`](http://wiki.openstreetmap.org/wiki/Tag:water=river) tag are classified as river.  Wet and dry docks
           tagged [`waterway=dock`](http://wiki.openstreetmap.org/wiki/Tag:waterway=dock) are classified as a `dock`.
+          Various minor waterbodies are classified as a `pond`.
           Swimming pools tagged [`leisure=swimming_pool`](https://wiki.openstreetmap.org/wiki/Tag:leisure=swimming_pool) are classified as a `swimming_pool`
           All other water bodies are classified as `lake`.
       values:
@@ -34,6 +35,8 @@ layer:
           waterway: 'dock'
         river:
           water: 'river'
+        pond:
+          water: ['pond', 'basin', 'wastewater']
         lake:
         ocean:
         swimming_pool: