Weighted Storage Cube Gears

Slightly modified the OpenSCAD file as I wasn't happy with the way the gear teeth intersected the cube - the 'horns' on the large gears in the original were very fragile and didn't look great.

This involved changing the gear teeth counts (from 18/9 to 18/12) and adding a 'rotation_tweak' parameter to the code that lets you adjust the rotation of the gears, so adding 5 degrees (half a tooth) moved the 'horns' from the large gear to the small ones.

As I don't have MatLab, I rewrote the gearo