OpenSCAD NPT / tsmthread 0.4

This is a fast and flexible thread-profile generator for OpenSCAD.
* It makes frugal, consistent, sound meshes.
* It makes undistorted NPT theads!
* It obeys $fs/$fa/$fn.
* It uses arbitrary profiles - i.e. the difference between UTS and ACME is a variable.
* It does multi-start threads.
* It can compensate for printing steep pitches like ACME.

As usual, all this stuff also lives on my site: http://burningsmell.org/3d/lib/tsmthread/

TL/DR
````
include ;

// 5/8" 11TPI imperial threads, 3/4" long, ordinary thread profile
imperial() tsmthread(DMAJ=5/8+$OD_COMP, PITCH=1/11, L=3/4, PR=THREAD_UTS);

// TR30x3 trapezoidal threads
translate([50,0]) tsmthread(DMAJ=30+$OD_COMP, PITCH=3, L=59, PR=THREAD_TRAP);

// TR8 thread, 2mm pitch, 4 start
translate([0,50]) tsmthread(DMAJ=8+$OD_COMP, L=10, PITCH=2, PR=THREAD_TRAP, STARTS=4);

// Experimental: compensated leadscrew threads. Angle and depth must be specified.
comp_thread(DMAJ=9+$OD_COMP, L=5, PITCH=2, STARTS=4, A=