Duetwifi and Duex2/5 enclosure

This version of "Duetwifi and duex2/5 enclosure 1.3" is an improvement on 1.2 and resolves a few issues and provides more selection options in the scad file.
With the duex2/5 selected the fan is a 40mm model and fitted internally or externally onto the side of the enclosure in order to blow cool air between the 2 boards. With just the duetwifi a fan of 40,50,60 or 80 mm can be selected and is fitted internally or externally in the rear of the enclosure.
The lids are fixed by using self tappin