fischertechnik N20 Mini Power Motor system

Changelog:
* 2018-09-25: (1) Corrected an error in `Enclosure_grooved_v4.stl`, the groove was 5mm short, now it is correctly 25mm. (2) I added a version of the enclosure parts and drilling aid for 4mm *square* brass rods, which are harder to get (where I live) but are much easier to cut and drill. Look for the files named `...v5_squareRods`, images at the end of the gallery.
* 2018-06-13: pinion & gear adapter now has filleted corners

This is an enclosure for the cheap N20 gear motors, whic