Customisable Calipers (two-colour)

This is a remix that fixes some of the issues with the original, such as the small scale on the bottom caliper being wrong (in the stl). It also cleans up some of the code.

Further, it changes the labelling to poke out of the base instead of receding into it. This allows the scale-lines as well as the numbers to be printed in a different colour by painting them.

**Version 1** has the problem that, after printing, the scale (a) is not easy to read and (b) breaks easily. Also, it was printed with a 0.4 nozzle, which was too coarse so the labelling is crooked.

**Version 2** adds a bottom depth probe and slightly increases the thickness of top-lines to improve readability. The depth probe is printed solid by choosing high wall loops.

Note: Please be aware that this design intentionally does not include a [vernier scale](https://en.wikipedia.org/wiki/Vernier_scale) because typical printers and typical materials are not precise enough to allow for sub millimetre accuracy.