Spring Mandrel V2: Spring Making Guide

This is a new version of my previous, never published spring mandrel / spring making guide with several improvements over V1

It is used to produce very small springs out of **music wire** for self-made solenoids.

### Libraries
The old version relied on Douglas Peale's [spiral_extrude](https://github.com/kr15h/openscad_spiral_extrude/blob/main/spiral_extrude.scad) library which renders incredibly slow (like glacially slow).

Version 2 passes to Phil Schatzmann's [spring implementation](https://gist.github.com/pschatzmann/1bf4617ff8543016333a3881d6522912#file-springs-ipynb) which renders way faster.

### Formulas
I also found several different spring calculation which turned out to be unreliable for small springs (giving negative lengths for mandrel diameter (?!??!)). They all seem to rely on " 'Helical Springs' by Kozo Hiraoka, in Home Shop Machinist, May/June and July/August issues, 1987" but I couldn't find an original source. I ended up estimating the mandrel diameter and app