Pen holder remake

As I wasn't quite happy with the pencil holder parts of the original https://www.thingiverse.com/thing:4688821, I remade that part. My major gripe I had with those was that the maximum allowed pen diameters were too small for many of my pens. I then also removed the overhang of the top mounting block, and removed the threads from the mounting holes, as I was using self-cutting plastic screws anyway.

While the .scad file has been provided, allowing customisation of other aspects than the pencil count (which directly affects inner diameters), a set of .stl files for various pen counts has been published too.

The inner diameters for the pre-made .stl files for pens they can accommodate are:
* pencils12: 8.5mm
* pencils11: 9.36mm
* pencils10: 10.4mm
* pencils9: 11.67mm
* pencils8: 13.25mm
* pencils7: 15.29mm

Variable n in the scad file is what has been changed for each of those .stl files, reflecting the pencil count.