Parametric E3D clone / Jhead / Hot end mount with proximity sensor

Designed for an E3D hot end clone, with a proximity sensor. There's also plenty of room between the hotend and prox. sensor for almost any fan you want to clip onto the hot end.

The following (among other things) are easily changed in the SCAD file:
Hot end heatsink diameter (if you have a genuine E3D)
Hot end height
Prox. sensor diameter, (if you have a 18mm prox. sensor)
Distance between the hot end & prox. sensor (if you need more room for a fan)
Mounting hole width (currently s