Rework Reprap Ormerod (MGN rail and z-sensor)

Around 9 years ago I modified my first 3D printer with an extra fan for the material (https://www.thingiverse.com/thing:873875). After buying and building some other printers, the Ormerod was waiting as an "organ donar" for few years. Than I found a cheap clone of the IR Z-senor and was curious, if this would have been really better than the original one. So I started to repair broken parts and modified the holder of the hotend to mount the sensor. It worked much better - but also make visible that the hot end walks like a "bow" when passing the feeder. Instead of stopping, I had the idea of replacing the original rod with a MGN12 - again just to see, if the result becomes better - this was around 1.5 years ago....

Now it's finalized, it works and the printer can go back into retirement :-)

The two coolermounts from https://www.thingiverse.com/thing:873875 are modified to fit with the dimensions of the MGN rail.
fan_v1_7.scad, fan_v1_7_HotendCooler.stl, fan_v1_7_MaterialCooler.