Geisha -- singular algebraic surface

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

Two singular lines, and two singular points on those lines!

I have provided these files:
* `geisha_thickened.stl` -- has the normal vectors fixed, and is thickened for printing.
* `geisha_raw.stl` -- raw triangulation coming from Bertini_real. Since the program works in arbitrary dimensions, I make no effort to control normals from it -- they d