Taurus Z axis for Prusa MK3 and MK3s

I didn't want the Z axis to miss out on the Taurus love, so I gave it a makeover. It's like viagra for your Prusa. Part of Z is already included in the Taurus X since that is the nexus of the two. I made this one more modular so you can upgrade parts or in stages. It comes with the obligatory overt excess, some fixes / innovations, and a dash of biomimicry. Here is what is changed:

* optional cross members to stiffen top of Z frame
* reinforced Z top with improved smooth rod coupling
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