Generated Retraction Tower

# Summary

A Python 3 program to generate custom retraction tower OpenSCAD, STL
and G-code files that can be used on a 3D printer to help in choosing
of a retraction setting that will reduce stringing.

WARNING: If you are not comfortable with running commands from the
command line, this tool may not be for you.

## Usage

NOTE: This has only been tested on a Prusa i3 MK3 printer using
slic3r-prusa3d (the Prusa version of Slic3r). It may not work
elsewhere.

* Run retraction-tower.py to generate retraction-tower.stl and
retraction-tower.gcode files (you can ignore the
retraction-tower.scad file).

```sh
python3 retraction-tower.py --levels 9 --start 0.5 --step-size 0.25
```

* Load the retraction-tower.stl file into slic3r-prusa3d.


* Enable the "Use firmware retraction" option found in the "General"
area under the "Printe