Magnetic SD card carrying case

Folding SD card case, holds 12 full-sized SD cards. Tool free and hardware free assembly (pivots are 1.75mm filament), but magnets are nice to keep things shut and provide a nice snappy action.

This design took a few cues from [ChargedNeuron's SD card case](https://www.thingiverse.com/thing:1320214), including the bifold design and the use of magnets. It's a bit simpler to construct and doesn't have room for a USB, however.