Customizable drawer assortment boxes system

A minor remix of the awesome openSCAD customizable drawer boxes by [1spire](https://www.thingiverse.com/1spire/designs). I only updated it to allow for flat inside bottom on boxes with a grid cutout on the outside bottom.

All you have to do is give the box bottom variable B_BT a height more than that of the G_rad. Ideally it should be G_rad+ wall thickness you want but if you're feeling adventurous you can play around.