Customizable Sierpinski tetrix

Continuing the theme of [fractals that print with their own support structures], this is a model of the [Sierpinski tetrix], a fractal based on the tetrahedron. It's a difficult object for a fused-filament printer because it involves lots of bridges, and the individual tetrahedra are only lightly connected to each other.

To make this model I started with [cgranade's] OpenSCAD [Recursive Sierpinski Tetrahedron], which includes a clever adaptation for 3D printing, a parameter that adjusts the adhesion between adjacent tetrahedra. It makes the tetrix less like the mathematical ideal, but a perfect model isn't printable anyway.

I added external support that lets the model print upside-down, which solves the bridging problem. See the instructions for how to customize it and suggestions on how to get a good print.

[fractals that print with their own support structures]: http://www.thingiverse.com/thing:240739
[Sierpinski tetrix]: http://mathworld.wolfram.com/Tetrix.html
[cgranad