Bell-mouth fan inlet

There are some excellent fan ducts for the Printrbot - such as [this one](https://www.thingiverse.com/thing:356001) - that improve considerably on the default duct. However, every one I've used seems to suffer from the same problem, e.g. the fan is trying to push more air through the duct than it really can handle. Part of the problem is that the flow *in* to the fan is in no way helped by the shape of the fan and how it is mounted.

I decided to try a classic bell-mouth inlet and center body; this is a common strategy to improve flow for wind tunnels and other systems where efficiency and smooth flow are important.

My non-quantified observations (putting my fingers up next to the inlet; listening to the flow; feeling the airflow coming out of the duct by the extruder tip) indicate that using this inlet helps quite a bit!

Even if it didn't make any difference at all to the performance -- it makes your Printrbot look *awesome* :-)

You will need slightly longer M3 screws to at