Z Axis Restraint for Pangu i3

Restraining the Z axis threaded rods at the ends can help reduce forces caused by wobble in the threaded rods being communicated down the X axis to the print head.
When taken in addition to other measures such as [thing 1077863](https://www.thingiverse.com/thing:1077863) Z axis wobble can be virtually eliminated.

Inspired by [thing 9864](https://www.thingiverse.com/thing:9864) this is a Z Axis constraint for the Pangu i3 variant of the Prusa i3 marketed by rp3d.com.

Pangu i3 has the Z ax