Pen stand (in OpenSCAD)

I needed a place to put my pen on my desk - I had designed [another pen stand](https://www.thingiverse.com/thing:6946875) previously, but putting a biro nose-down for any length of time leads to blobs of ink at the tip.

So I decided that a 'lay-flat' design was more appropriate. Looked for any suitable design to save time, but most seemed to be geared for fountain pens, so were too big. This one is compact, and just 'big enough' for the pen and pencil I use daily.

I put 4 self-adhesive non-slip plastic feet on mine, but these are optional.