Add building heights (#56)

* Add better support for 3d buildings

Change key from "levels" to "building:levels" as the "levels" key is non-standard and sees much less use than "building:levels"

* Fix typo in sql definition and missing render_min_height

* Add render_height and min_height to building.yaml

* Simplify the render, switch numeric to int.

Assume min_height cannot be negative. There are a few examples in overpass, but can't show below ground anyways in mapbox (https://github.com/mapbox/mapbox-gl-js/issues/3456). Also don't attempt to fix min_height>height

* Remove colour as it is not currently used

layers/building/mapping.yaml

@@ -20,10 +20,18 @@ tables:
     - name: height
       key: height
       type: integer
+    - name: min_height
+      key: min_height
+      type: integer
     - name: levels
-      key: levels
+      key: building:levels
+      type: integer
+    - name: min_level
+      key: building:min_level
       type: integer
     mapping:
+      building:part:
+      - __any__
       building:
       - __any__
     type: polygon