ST_Delaunay enhancement

[ocourtin]

GEOS implementation provide two extra features:
-> Tolerance value to snap input vertices together
-> ability to choose output type among (COLLECTION of POLYGON, LINESTRING, TIN)

Aim is to have the same with SFCGAL

cf http://postgis.net/docs/manual-dev/ST_DelaunayTriangles.html

[mhugo]

We don't know how to snap round in 3D.
ST_DelaunayTriangles is closed to the function discussed in #36