Ding dong - singular algebraic surface

Ding dong, a singular algebraic surface of degree four. This is the set of real points for which
```
x^2 +y^2 +z^3 - z^2 = 0.
```

Has a single singularity, a bounded blob, and an unbounded skirt!

I have provided these files:
* `dingdong_thickened_smooth_fixed.stl` -- has the normal vectors fixed, and is thickened for printing.
* `blocky_fixed_2.stl` -- the blocky version, good for printing. 2, because 2.
* `dingdong_raw_smooth.stl` -- raw triangulation coming from Bertini_real. Since the program works in arbitrary dimensions, I make no effort to control normals from it -- they don't exist for 4- and higher-dimensional surfaces, but instead a tangent space which is not immediately useful for 3d printing. Hence, the surface was thickened in Blender, before being passed through Microsoft's 3D Builder to fix internal geometry. The raw versions are not directly suitable for 3d printing. Infinitely thin, ran through `sampler` with fairly tight tolerances.
* `input` -- the