Yet another thread library for OpenSCAD

This is yet another OpenSCAD thread library, and it was more fun to write it than to find a library that did exactly what I wanted. It's designed to be fairly fast: it generates the whole thread polyhedron as a single piece.

Thread profile is completely flexible. You can specify a counterclockwise polygon of 2D points for the profile or you can use ISO thread profiles.

There are two modules exposed:
`rawThread(profile, d=diameter, h=height[, lead=lead, $fn=segmentsPerRotation])`: draw thread with specified profile; if `lead` is omitted, it is calculated from the profile
`isoThread(d=majorDiameter, h=height, pitch=pitch[, angle=angle, internal=true/false, lead=lead, starts=starts, $fn=segmentsPerRotation])`: draw metric ISO thread; default angle is 30; default is external thread (male); internal thread is designed for subtraction
If you want to use UTS measurements, you can omit pitch and instead do `tpi=TPI`, and then use `hInch`, `dInch` and `leadInch` instead of the metric