Renkforce RF100 fan duct

###Size
Thing size is 77 x 40 x 10 mm3.

###Summary
This is a modified version of the fan duct proposed by Oliver_D. The main modification is
* The air flow is now following Bezier based shapes in the hope that it will facilitate air flow.
* Output is now single sided in the hope that air speed on the object will be faster, facilitating cooling.

For the rest it is a copy of the original by Oliver_D.
* A small improvement in the upper-front hole which is made wider as the screw is alwa