MakerFarm i3v Z rod support - Improved

This improves on kkessler's Z axis protectors by adjusting various dimensioning such as hole spacing and hole sizes, allowing for a corner radius that better matches the MakerFarm style, and by providing additional options regarding use of a bearing.

Only a single support bracket design is provided that is intended to be used on both Z axis threaded rods. I don't plan to provide a version that includes mounting provisions for the MakerFarm top-side spool mount.

The openSCAD file allows m