Pi Zero4U case

The Zero4U case is a two-part case designed for the pi zero 1.3 with the [Zero4U](https://www.adafruit.com/products/3298) USB hub. The case uses a snug fit to hold the Pi rather than the screw holes (as they are already in use by the USB hub).

I have also included the step file for making modifications

Edit: I separated the single step file into two, one for each half.