Example OpenSCAD Screw Thread Transformation

Inspired by [speedymollusc's openscad screw threads](http://www.thingiverse.com/thing:368073), this is an example function for transforming a "radial" polygon representation of screw threads into an "axial" polygon suitable for linear_extrude with rotation.

By "radial" representation, I mean a list of coordinates [[ r1, z1 ], [ r2, z2 ], ... ] where each pair contains a radius and z position (dimension parallel to axis of screw). Such a polygon will generate a thread-like shape when used with rotate_extrude, but unfortunately OpenSCAD does not allow "rotate_extrude with advance" to produce screw threads this way.

If a screw were cut perpendicular to the axial direction, the shape of the cross section would contain the information about the thread shape, but in a different (and less intuitive) form. This I am calling the "axial" polygon. This polygon, when used with linear_extrude with twist, can generate the thread shape.

Contained in this short .scad file is a pair of func