Raspberry Pi 4 Case - 40mm Fan

Borrowed completely from [219Design's excellent original](https://www.thingiverse.com/thing:4103385 "Snapfit Raspberry Pi 4B Case"), with the following changes:

**Lid**
* countersunk screw holes and grate for a 40x10mm fan

**Bottom Shell**
* Increased thickness of side walls to ~3mm
* Added 2mm to height/clearance in the base
* Recessed the USB-C power port to provide reasonable fit with revised shell thickness
* Add another set of vent holes opposite original side vent
* Removed recessed mount holes on bottom
* Removed retainer nibs from board mounts

My original intent was simply to add support for a 40mm fan, but I was not enamored of the clip-in mount (and frankly was not sure I could replicate it with the skill 219Design supplied ... I'm very new to this). And when I printed the original base, the shell seemed a bit flimsy, but that was more likely the fault of my printer than the design. Thickening the walls took care of that issue.

The nibs for securing the cir