nut-trap C-clamp, semi-customizable, captive nut

This is a customizable C-clamp that uses a nut and screw for the clamping force, much like a "[beam clamp](https://www.google.com/search?q=beam+clamp)"

It rounds the inside corners and outer edges to reduce stress concentrators and susceptability to cracks, to save some material and time with the excellent [Round-Anything](https://github.com/Irev-Dev/Round-Anything) library.
Unfortunately, this code is only customizable outside of Thingiverse's OpenSCAD v2015.3.0 customiser. I can't seem to make the code work with here without a segfault. Do your customizations outside of thingiverse in OpenSCAD.

You could use a [customized handle](https://www.thingiverse.com/thing:5569782) on the clamp screw to dress it up.

The nut parameter list was inspired by Nophead's NopSCADlib nuts.scad file: https://github.com/nophead/NopSCADlib/blob/master/vitamins/nuts.scad

A small, unobtrusive clamp has many uses.

2022-10-16 -- Updated with easier to select nut trap sizes M1.6-M12