Powerbank for M5stack Atom Matrix

This is a rechargeable 400mAhr powerbank for the [M5stack Atom Matrix](https://docs.m5stack.com/en/core/atom_matrix), designed to be FDM-printable and serviceable (no glue!).

*Warning*: I noticed this warming up while charging, so not yet convinced it is safe for extended charging. I haven't noticed heat issues while in use / discharging.

This needs to be printed with a transparent filament so you can see the charge lights of the internal charging board. Best to print the "side" part on a raft. The spacer and bottom corners needed a little sanding for fit and flatness. Wrap the battery in kapton tape to prevent electrical contact with the charge board. The 4 screws that hold the sides on were heated with a mini blow torch so they could melt their way in, rather than splitting the thin edges.

Parts list:
* [400mAhr lithium battery](https://www.aliexpress.com/item/1005004779952950.html)
* [USB C charger board](https://www.aliexpress.com/item/1005005108273423.html)
* [USB C ma