Raspberry Pi Zero model

I've heavily revised this thing.
- Rewritten the code now that I understand vector variable types better.
- Corrected the number of GPIO pins (now 20, was 21 )
- Added zeros 1.3, W & WH. Code will be a little easier to maintain too when the next zero model gets announced.
- "ACT" LED now shown (should you need it for a case cutout).
- Option to show GPIO pins colour coded.

- External connectors remains on the "to do" list.

An OpenSCAD model of the Raspberry Pi Zero that I've made t