Simple BluCon holder

Please also check version 2 below.

This is an attempt to build a simple and practical support to hold a BluCon reader on a Libre patch. The ring holds tight enough on the Libre and the four holders keep the BluCon nicely in position but lets you remove it with one hand. Not much testing has been done, but it seems to work.

I also included the FreeCAD project to make it simpler to adapt to other printers in case it doesn't fit perfectly. The number of holders can be increased in the polar a