Improved Anet A8 Center Nozzle Fan Duct

**According to the comments, this no longer fits Anet A8 blowers due to a smaller blower fan opening. I'll see about uploading a second design.**

This is a from scratch remodel of the "Hesine M-505, Anet A8 - Center Nozzle Fan." I needed to make a modification of the fan for a dimensional change on my printer. In the process, I attempted to make some design changes to improve airflow and decrease noise:

* Thinner walls near the fan output to increase airflow and decrease noise.
* Smoother