Alphawise u20 tool holder

**Disclaimer:** this is my first project! :)
This is a remix of this [CR10 tool holder](https://www.thingiverse.com/thing:3034028) adapted to Alphawise u20.
Only the legs (including the leg allowing to carry a raspberry pi) have been enlarged to fit the u20. The tool holder itself is unchanged.

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**Edit Jan 13:**
- Added some buttress to reinforce the legs as my first print using ABS broke there.
- Also added a big buttress below the hangover to try to get rid of support below the hangover of the PI leg
- Remade the tool holder itself from scratch for U20 with a new hole for the nozzle cleaning pin and 3 places to store SD cards