Tolerance test

Designed as a test of tolerances for 3D printing.

Inspired by my own (mis-)adventures learning the CAD (Onshape), slicer (Cura), and 3D printer (Monoprice Maker Select Plus) workflow - and also a Maker's Muse episode:
https://www.youtube.com/watch?v=TYuLVN3YHw8

To do this exercise properly, you need to update parameter(s) in the model to match your printer (which is really **not** how this should work, but).

Will write this up as a weblog post, at some point...

In my latest (versioned in Onshape) iteration, got down to 0.2mm in gap between parts, with a parameter in the model, which ... is quite wrong. The parameter should be part of the printer profile given to the slicer. Still.

**From the Document Description (in Onshape):**
The #overfill parameter is used to adjust the amount lines of plastic laid down by an additive 3D printer are a bit wider than where the plastic is meant to land. For example, a 12mm circle (in the x-y plane) when printed on my current printer m