Attend to small shape bugs in the z-axis structure

Issues #310 (closed) and #311 (closed) identify small bugs in the way the shape of the z-axis structure is defined.

This MR:
Makes the changes noted in #310 (closed) for the actuator cut-out in the screw_seat()
Makes the z-axis casing symmetrical by centering a cube in the structure.

closes #310 (closed)
closes #311 (closed)