Thicker STEMFIE screws

I am changing the #Stemfie3D screws, making them thicker (4 to 5 mm). This change makes all fasteners the same thickness, including shafts and spring pins.

This allows for an internal axial hole and makes them stronger and stiffer; it also gives the screws a larger thread surface and reduces the play inside the part holes. 

You can expect the old 4 mm fasteners to be phased out and the new ones introduced in the near future. All the latest files are available at get.stemfie.org.