Parametric thin wall test object

This is a thin wall test with both rounded and sharp corners. My slicer says it takes about 0.29m of filament, so it's a cheap test for adhesion, elephant-foot, speed, retraction, cornering, dimensions, thin-wall extrusion, and quality.

You can set the wall thickness to be your nozzle's ideal extrusion width. These three stls have different wall thicknesses:

| part | wall mm |
|-----|-----|
|thinwall_20_30_10_0p50.stl | 0.50 |
|thinwall_20_30_10_0p60.stl | 0.60 |
|thinwall_20_30_10_1p00.stl | 1.00 |

I printed the pictured rough white part back in 2012 on a home-built [Wallace](https://reprap.org/wiki/Wallace) with my own version of a [Wildseyed MIG Welder Tip hot end](https://reprap.org/wiki/Wildseyed_Simple_Hot_End#Step_Three_-_The_MIG_Welder_Tip), and wanted to redo the test on my new printer, but didn't have the pattern. So I hacked up the green one based on https://github.com/Irev-Dev/Round-Anything :

```
// thinwall_20_30 -- thin wall test piece, copied fro