OpenSCAD Gear Library with Customizer

This is a remix of Jörg Janssen's "OpenSCAD Getriebe Bibliothek" (OpenSCAD Gear Library).

_(For instructions on how to use this thing, see bottom of this page.)_

Supported are:

* Rack
* Spur gear¹
* Herringbone gear¹
* Rack and pinion¹
* Annular/Internal spur gear
* Annular/Internal herrinbone gear
* Planetary gear
* Bevel gear
* Bevel gears (meshing pinion and idler)
* Herringbone bevel gear
* Herringbone bevel gears (meshing pinion and idler)
* Worm
* Worm drive (meshing worm and worm gear)

¹ With optional gear hub

Changes:

* I have added English customizer variables for all the gears.
* Grouped parameters per gear type
* Added customizable hub option for spur and herringbone gears

Todo:

* Add an option for double gears.
* Add an option for a spur gear drive (two meshing spur gears).
* Add an option for gear box generation.
* I like the library as it is, but the perfectionist in me wants to add a proper trochoidal section to the gear tooth