Voronoi vase

Although some modeling tools can easily create a Voronoi vase, using [OpenSCAD](http://www.openscad.org/) to create it is a meaningful challenge. I use the `voronoi3d` module of [dotSCAD](https://github.com/JustinSDK/dotSCAD).

The preview or rendering of 3D Voronoi is slow so the source code doesn't create the Voronoi vase directly.

The code here requires OpenSCAD 2019.05. You have to download the scad file, use OpenSCAD to render and export the 3D Voronoi model first.

Then, update the `stl_file` in the scad file to the stl filename. render and export the final model.