Prusa MMU2 selector mod

**update 8/28/19** I updated this to mount the spare 10x6mm magnet from Prusa under the FINDA ball. This was done by others, but I missed the windows I added. I often find debris is self ejected from the window which is nice. I also added garbage chute to rear and cleaned up some aspects of print. You may need to clean up magnet slot some to fit.

**update 8/30/19** couple tweaks and added back missing chamfer for cleaner print (bridge detection). Added step.

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MMU2 is a finicky b