WIP retractable pen

**Very much** a **work in progress**, ImplicitCAD does not seem to output `.stl`/`.obj` files that slic3r likes. Also I dont think the pen would grab the lead well.

`pen.stl` provided went through meshlab using the 'Surface reconstruction poison' remeshing with octtree depth 10. (..Which isnt perfect, it changes the surface, i think )

Slic3r does some funky things(it might be the model) at the bottom, probably want to watch the start of the print, so you know it doesnt come off.

I was a