Parametric Spur Gear

A parametric spur gear, made with the involute curve tangent line [approximation](https://nptel.ac.in/courses/116102012/40), as Fusion 360 doesn't support equation driven curves (yet). Module, number of teeth, height, hole diameter, and pressure angle are adjustable parameters. See the [parametric gear toy](https://www.thingiverse.com/thing:3336663) for a test model using these gears.

GitHub: https://github.com/vsergeev/3d-gears/tree/master/parametric-spur-gear

Design on A360: https://a360.co/2BSMisY

### Usage

Standard ISO spur gears are specified by two top-level parameters: gear module and gear teeth. Gear module is the pitch of one tooth on its reference diameter, and should be equal between interacting gears. Gear teeth are chosen to achieve the desired gear ratio between interacting gears. These parameters are exposed as `GEAR_MODULE` and `GEAR_TEETH`, respectively.

The reference diameter of a gear is computed with `GEAR_MODULE*GEAR_TEETH`. With this design, interac