Raspberry Pi B+ / Pi 2 clam shell case

Another Raspberry Pi B+ / Pi 2 case. The differences to all those _great cases out there on Thingiverse_ are:

- Self centering clam-shell design: Light will not shine though the gaps, robust enclosure
- No lateral space limitation for heat sinks
- Additional round 5mm hole for some GPIO cables
- Combines features of other cases:
- The holes are well fit within an intentional tolerance of 0.5mm
- Well tightened by common 10mm M3 screws and nuts
- By using longer screws case can be stacked - see i.e. "usb_hub_shell" for DA-70215
- Two holes for 2mm light gathering acrylic rods
- Should be well printable without supports
- Added a version with two M3 mounting holes and 70mm separation

If you like to add your own holes, mounting options or like to close the venting grid, I have provided the corresponding STEP files.

__At this point a friendly reminder for Pi Owners:__ _There are different USB cables out there. Watch out for the AWG number which is related to the di