Redo ST_ConcaveHull utilizing Delaunay Triangulation

[robe]

At the time I wrote ST_ConcaveHull, we didn't have ST_DelaunayTriangles so its much less efficient than it should be. ​http://www.postgis.org/documentation/manual-svn/ST_DelaunayTriangles.html

The next version I was hoping to incorporate that logic, but that may require I have two versions since some people feel we need to support many versions of GEOS for each instance of PostGIS and I can't therefore require GEOS 3.4.0 without breaking backward compatiblity for some folks who are unfortunate enough to run 2.1 with anything less than GEOS 3.4.

Next iteration, I'm planning to follow the other Sandro's approach as noted here:

​https://www.gaia-gis.it/fossil/libspatialite/wiki?name=tesselations-4.0

As a side note - it would be nice if we had all those tessslation functions that Spatialite has as well since they are a common step in many workprocesses.