cleaned up parametric belt clamp, with customisable screw sizes.

i like the original clamp from neurofun, but it requires other scads as dependencies. so i looked at the remixes and found one (by andygock) that removed the dependencies to the original source files from prusa. this one had the required dependency code put right into it, but still uses m3 screws and my machine uses m4's for the belt clamp, and did not support changing this without knowing what to change.

so i cleaned up the code, put the variables neat the top, added a few variables (screw d