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.

---

MMU2 is a finicky beast.

I added pockets to the selector around the finda ball so that there is room for debris to get out of the way of blocking the ball, which is my number one fail mode.
I added windows so you can:
* monitor filament loading
* observe the ball status
* set the FINDA height visually
* **extract debris mid-print**

recommend print at 0.15 layer to get smoother transitions on filament path.


**To install** just follow the directions for changing the blade which requires removal of