Parametric GT2 Pulley

A parametric pulley for 2mm pitch GT2 timing belts. Number of teeth, face width, lip thickness, outer diameter, and bore diameter are adjustable parameters. See the [belt demo](https://www.thingiverse.com/thing:4304834) for a test model using these pulleys.

GitHub: https://github.com/vsergeev/3d-gears/tree/master/parametric-gt2-pulley

Design on A360: https://a360.co/2KkRSb4

### Parameter Summary

* `GT2_PULLEY_TEETH` - number of teeth (default 20)
* `GT2_PULLEY_Z_FACE_WIDTH` - face width (default 7mm)
* `GT2_PULLEY_Z_LIP_THICKNESS` - lip thickness (default 3mm)
* `GT2_PULLEY_XY_OUTER_DIAMETER` - outer diameter (default 15mm)
* `GT2_PULLEY_XY_BORE_DIAMETER` - bore diameter (default 5mm)