Compound Planetary Gear

Compound planetary gear.

Inspired by [Gear Down for What](https://www.thingiverse.com/Gear_Down_For_What) and [GDFW youtube](https://www.youtube.com/results?search_query=gear+down+for+what).

I've also made a openscad module to make [compound planetary gears](https://github.com/orloffsr/PolyGear), which uses the [PolyGear library](https://github.com/dpellegr/PolyGear). It's not terribly high quality coding style but it seems to work.

This particular compound gear has sun and planet teeth of:
s=[11,15,11,12], p=[13,11,13,14], which, with the first and third rings stationary, gives a ratio of 28.4 and 1134.5 for the second and fourth stages. This was also made with an earlier version of the openscad code so some details differ. However, it printed and worked fine--tested with a drill.

Included in the github files is a python function that will do a comprehensive search for gear ratios and find number of sun and planet teeth that match given criteria.