Spherical Involute Bevel Gears library for OpenSCAD

This is super-alpha preview code. Currently requires OpenSCAD 2019.05 due to use of the new "for each" list comprehension features. May get backported to 2018 later. Please report any obvious bugs in basic usage. Almost certainly does not work if pushed to more exotic use cases like 'inner' bevel gears, or extreme angles. I have not even printed any gears yet, let alone used them in real-world projects.

Open the file, use the customizer to play with options. Look at the "bevel_gear_pair_test()" module for ideas.