pd-gears-v2.0.1: fix rack with mm per tooth < 9

• rack was broken on mm_per_tooth < 9
  dcf66ea

pd-gears-v2.0.0: pack for OpenSCAD Modules registry

Start making changes to original Public Domain gears starting from verstion 2.0.0:

• add 'center' [true/false] param to gear and rack modules to decide if gear or rack should be centered by z axis, don't center by default
• add $fn param to gear module to control number of fragments when render hole cylinder, $fn=20 by default
• packaged for Openscad-Modules https://github.com/RobertFach/Openscad-Modules
• minor internal code fixes and improvements

pd-gears-v1.1.0

Based on project by Leemon Baird http://www.thingiverse.com/thing:5505 with some minor and major API fixes and improvements.

Version 1.1 is total copy of publicDomainGearV1.1.scad from thingiverse project, modifications start from 2.x.