Introduced sub-classing of CadQuery Shape:
- enhanced the cadquery core to support sub-classes of Shape
- converted Nut, Screw, Washer, Thread, and Sprocket to sub-classes of Solid
- converted Bearing to sub-classes of Compound
- deprecated use of attribute cq_object for sub-classed objects
- updated documentation, examples, and unit tests
- added user documentation for custom sub-classes