Screw library for Openscad

This screw library takes a different (simpler) approach to making threads. Instead of drawing threads on a cylinder, it creates a stack of circle(ish) discs that represent a slice from a screw. Similar to the stack of shapes that a 3D printer slicer makes, and that actually gets printed. In this case they are created in thin pairs, then wrapped in hull() so they display like a regular screw. Note: a full article / tutorial will be coming shortly describing this method in detail.

Four screw profiles are defined -- profile 0 is for making nuts (for use in combination with the difference() operator, and profile 1 is a standard screw. These are profiles that follow the ISO metric screw standard shape. But they end up not screwing together too good unless you make the bolt smaller, or the nut larger, as the 3D printed layers (at .10 mm) are jagged and bind up. So two more profiles are supplied, and recommended -- profile 2 is for nuts, and profile 3 is for screws. These define sc