Parametric seed box

Parametric small rack ("seed box", "Setzkasten") e.g. for storing
miniatures or other collectibles.

Small racks might be printed on a 3D printer, but the main use
is to have the parts cut using a lasercutter.

You can parameterize:
* thickness of material,
* inner width/height/depth of individual boxes and
* the number of boxes in horizontal and vertical direction.
* length of the keys/slots

This third version fixes all known bugs from previous version.