improved Gear Shape for parametric_involute_gear_v5.1.scad

Wnen using "parametric_involute_gear_v5.1.scad", I've had several problems with smaller - the teeth were breaking off. I looked at the shape of the join between the hib and each gear tooth - a very shap edge - pretty much anathema for strong FDM printing - so I added a fillet to each tooth edge. Serendipitously, the gears mesh better, feeling much feel smoother when rotated by hand.

I added a parameter gearFillet to module gear(), my main code additions start at line 430. The parameter defaul