FilletBox as a solid-like object instead of a Solid subclass

[fpq473]

See examples/fillet-box.py for FilletBox following a "SolidLike"
protocol in order to be treated like a solid by Workplane.

See cadquery/cq.py for an example of the kind of change that Workplane
will need to support SolidLike. Currently only Workplane.union() has
been adapted. This is the Workplane method demonstrated in
examples/fillet-box.py.

[fpq473]

And pasting the output of examples/fillet-box.py:

fillet_box=<__main__.FilletBox object at 0x7f1811a92f10>
fillet_box.translate((1, 2, 3))=<__main__.FilletBox object at 0x7f1811a92eb0>
fillet_box.scale(2)=<__main__.FilletBox object at 0x7f1811a92eb0>
Solid(fillet_box)=<cadquery.occ_impl.shapes.Solid object at 0x7f1811a92eb0>
Workplane().union(fillet_box).findSolid()=<cadquery.occ_impl.shapes.Compound object at 0x7f1811a92ca0>
isinstance(fillet_box, SolidLike)=True
isinstance(Solid.makeBox(3,4,5), SolidLike)=True