OpenSCAD library candidate: torus

A rewrite of my torus library addition.

It makes use of this pie library function, https://www.thingiverse.com/thing:6754617, which has also been added to the files in this download.

The torus module can:
generate full tori (360°): torus(rad1, rad2);
generate torus segments: torus(rad1, rad2, angle);
generate torus segments with "blobs": torus(rad1, rad2, angle, rad3);
Those blobs are spheres added to both ends of torus segments. The hollow demo torus segment from the pictures above has such blobs with rad3 = rad2 added.

WIP because I'm still thinking of changing the first argument to specify ring center instead of inner ring edge.