Auto-rewinder for RepBox 2 mashup

My mash-up of the spool holders from @jterranella along with the auto-rewind mechanism from @VincentGroenhuis.

The STLs included in this Make are from version 5.4 of Vincent's spool holders. I would assume the newer versions would work even better.

I had to modify the existing axle to fit properly, so I've included the STL for that as well. It was needed to be able to fit 5 spools in the RepBox 2.

For assembly instructions and the required hardware bits, please refer to the two upstream source projects.