Added a cap for the upright foot based on @rootboi's (from forum) CAD diagram

A new cap for the top foot of the upright was designed on the forum by rootboi: https://openflexure.discourse.group/t/upright-microscope-prototype/587/28

They sent me this nice techincal drawing:

I have added parameters and turned it into OpenSCAD. Very slightly different dimensions. Probably should be tested on Bath and Glasgow's printers (@williamwadsworth @freyawhiteford)

Closes #357 (closed)