Columpius - smooth algebraic surface

Columpius, a smooth algebraic surface of degree four. This is the set of real points for which
```
x^3*y+x*z^3+y^3*z+z^3+7*z^2+5*z = 0.
```

Has two holes!

I have provided these files:
* `columpius_smooth_thickened.stl` -- has the normal vectors fixed, and is thickened for printing. I should re-do this one, because there are artificial self-intersections near one of the holes. I now know how to do this step better.
* `Columpius_Blocky_Xy_thickened.stl` -- the blocky version, good f