Print the bowden coupling as a separate piece

This avoids overhangs. Yay!
Hull the bowden trap nutholes downwards
Fix screwhole clearances
Add separate bowden trap
Add Makefile
Remove epsilon compensatoin on bowden_trap_screwholes
Drop bowden_trap() module from alublock-mount.scad

It's now in bowden_trap.scad.
Add assembly.scad
Tidy up bowden_trap code and add fillet
Add sideplate for right side