Prusa I3 MK3 Parametric OpenSCAD Remix

Originally I just wanted to change the position of the x axis idler, but the original Prusa OpenSCAD code makes it hard to change things because most dimensions are used directly. That's why I've decided to rewrite all parts, but driven by parameters stored in a single file.

[OpenSCAD code here](https://github.com/puddnig/Prusa-i3-Parametrized/tree/MK3/Printed-Parts/scad)

Currently I've rewritten the following parts:

* #####z-rod-bottom
The Stepper is centered as in R2 of the original