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. S