Bounded Voronoi Tesselation using the algorithm described in xlr8r.info
This is a variant of the Bounded Voronoi Tessellation, with:
circularly distributed sites
the median distance instead of the mean (allowing a small optimisation)
many more exterior control points
a d3.curveCatmullRomClosed convex hull shape
not displaying links and sites
Author: Philippe Rivière, August 2016
Based on mbostock‘s block: Voronoi Tessellation
forked from Fil‘s block: Circular Bounded Voronoi Tessellation