Improved Tantillus Herringbone Gears

Redrawn with several modifications to suit my particular build.

- Suit M4 captive nuts
- Gear teeth remodelled: 1.5 Module, 25deg helix angle, 41 teeth
- 3 locking screws so they can be better adjusted on the shafts
- More substantial hubs and webs for improved strength
- Flipped the driven gear so they both sit the same way round on the shafts
- Generally tidied up the geometry