cap - 9V battery

- 9V Battery contact protector.
The existing thingiverse models all seemed to be flawed. Either the fit was wrong, or they assumed force to remove the clip should be applied along the long axis of the protector...which i find - unless the cap is flexible - usually bends or breaks the contacts.

- Honourable mentions to:
- https://www.thingiverse.com/thing:261720
Not because I remixed it. Because I didn't. I printed it and was frustrated by it, so it became inspiration to make a new one completely from scratch. One that had tolerances appropriate to the two battery types i have (Energiser and BatteryWorld).
- https://www.thingiverse.com/thing:1941241
Because decent 'dummy' models are awesome. Especially when the relevant dimensions mostly conform to the measurements I was taking from my own batteries.

- Works for me:
- PLA, 0.4mm nozzle, 0.2mm layers
- 1.2mm thick walls/ceiling/floor assumed but not entirely crucial.
- 0.4mm external perimeter width specified in