Parametric Gear Toy

A parametric gear toy based on the [parametric spur gear](https://www.thingiverse.com/thing:3336641). The four gear teeth counts are adjustable parameters. The default settings yields a gear ratio of approximately 3 from the outside to inside gear. Three M5 threaded thumbscrews keep the gears in place on their shafts.

See the [parametric bearing gear toy](https://www.thingiverse.com/thing:3336671) for a bearing based version.

This design exposes the following top-level parameters:

* `TOP_GEAR1_TEETH` - tooth count of right gear (default 35)
* `TOP_GEAR2_TEETH` - tooth count of top middle gear (default 16)
* `TOP_GEAR3_TEETH` - tooth count of bottom middle gear (default 33)
* `TOP_GEAR4_TEETH` - tooth count of left gear (default 24)
* `TOP_GEAR_SHAFT_DIAMETER` - shaft diameter of all gears (default 7.5mm)
* `TOP_GEAR_HEIGHT` - height of all gears (default 5mm)
* `TOP_GEAR_SCALE` - tooth scaling for process margin (default 0.925)

This design addresses material shrinkage