small dc motor gearbox

This is set of objects and scripts allowing to build very simple gearbox.
I made it mainly for re-use of small and weak DC motors.
They are usually too weak to use with printed planetary gearboxes, so they require
very low-torque first stage.

Changing and choice of gears is very easy as gearbox core support is based on
two threaded rods.

It contains parametric (or at least easy to modify - openscad) motor holder ,
skate bearings holders (they are used to support rome reductor gear