Merge branch 'develop' of github.com:CartoDB/crankshaft into add-contour

doc/09_voronoi.md

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+## Voronoi
+
+Function to construct the [Voronoi Diagram](https://en.wikipedia.org/wiki/Voronoi_diagram) from a dataset of scatter points, clipped to the significant area
+
+PostGIS wil include this in future versions ([doc for dev branch](http://postgis.net/docs/manual-dev/ST_Voronoi.html)) and will perform faster for sure, but in the meantime...
+
+
+### CDB_Voronoi (geom geometry[], buffer numeric DEFAULT 0.5, tolerance numeric DEFAULT 1e-9)
+
+#### Arguments
+
+| Name | Type | Description |
+|------|------|-------------|
+| geom   | geometry[]  | Array of points's geometries |
+| buffer | numeric   | enlargment ratio for the envelope area used for the restraints|
+| tolerance   | numeric |  Delaunay tolerance, optional |
+
+### Returns
+
+| Column Name | Type | Description |
+|-------------|------|-------------|
+| geom  | geometry collection | Collection of polygons of the Voronoi cells|
+
+
+#### Example Usage
+
+```sql
+WITH a AS (
+    SELECT
+    ARRAY[ST_GeomFromText('POINT(2.1744 41.403)', 4326),ST_GeomFromText('POINT(2.1228 41.380)', 4326),ST_GeomFromText('POINT(2.1511 41.374)', 4326),ST_GeomFromText('POINT(2.1528 41.413)', 4326),ST_GeomFromText('POINT(2.165 41.391)', 4326),ST_GeomFromText('POINT(2.1498 41.371)', 4326),ST_GeomFromText('POINT(2.1533 41.368)', 4326),ST_GeomFromText('POINT(2.131386 41.41399)', 4326)] AS geomin
+)
+SELECT
+    st_transform(
+    (st_dump(CDB_voronoi(geomin, 0.2, 1e-9)
+    )).geom
+        , 3857) as the_geom_webmercator
+FROM a;
+```