f/threads: Update to file version 2.5

[rockstorm101]

Obtained from: https://dkprojects.net/openscad-threads/threads.scad

Fixes openscad/MCAD#79

[hyperair]

I think the two files have diverged significantly, so we shouldn't just blindly check-in the new version from openscad-threads. We can try to merge in some features, but otherwise, the library in MCAD should be considered a complete fork.

Some main differences that aren't easily reconciliable:

• The existence of the generic trapezoidal thread method
• The difference in implementation -- we use sweep() of a 2D object for a much more efficient implementation of threads, compared to manually moving a 3D polyhedron around. We also obey $fn, $fs, and $fa, which the other version does not.

[hyperair]

I'm closing this PR for now. Please open another if you wish to port the new features over from openscad-threads.

[rockstorm101]

Hi @hyperair, thank you for reviewing this. I agree with your comments, particularly:

I still don't think they should be part of the same module though.

and

we use sweep() of a 2D object for a much more efficient implementation of threads [...] We also obey $fn, $fs, and $fa, which the other version does not.

So I'm happy to close this. I'll try and work out (hopefully with Dan) how to re-implement all this.

EDIT: Did I miss any other module more mature to produce threads?