Adjust the motor lugs to improve the z-axis structure

The motor lug brackets were added twice for the z-axis:
First to build the hull() for the body of the structure.
Then again to put back the lugs and motor screw holes.

This meant that the brackets for the motor lugs could protrude into other cut-outs from the z-axis casing, particularly the access hole for the optics module screw.

This MR makes a switch for the brackets in the motor_lugs() module. That switch is then used so that the final addition of the motor screw holes on the z-axis has the cylinders of the motor mounts, but not the bracket parts.

Also a small change to neaten up the transition from the brackets to the motor mount cylinders, where the equator of an STL sphere of radius 5 is slightly smaller than a cylinder of radius 5, because of facets in the vertical section.

Part of #421 (closed)

Optics screw access hole, before and after:

There is a little gap now in the z-axis. This gives an overhang, but it is less than 1mm and the slicing is fine.
Before and after:

The motor lug transition, before and after: