USB Microscope Stand

All files are in .3mf (because its far superior to .stl). Thingiverse requires a .stl file though, so I added my retraction calibration .stl to the list.

Anyways, took ekaggrats .3dm file and heavily remixed it.
* Solid rack so that print is quicker with WAY less perimeters (plastic weight is approx. same when printed with 5-10% gyroid infill).
* Made a base that is solid with a 1cm square grid on it for alignment and size purposes.
* Added some ribs to the wheels just incase someone reefs on the wheel too hard and rips the layers apart as well as a depth stop that rides against the pillow blocks.
* Fixed the pillow blocks to be... not garbo. Pillow blocks and wheels now have a conical centering system with no tightening needed as friction is achieved via the rear of the rack.
* Extended the scope carriage out to be near the center of the pad (its not quite center... i need to fix that later).

All this because the base stand of my [microscope](http://www.amazon.com/gp/