Custom Fan Ducts for CraftBot with circular cross section air nozzle

This is a modified version of "CraftBot Custom Fan Ducts" from 3DForms, many thanks to him for the idea.

I am hoping that it will improve bridging performance of CraftBot.

Here is the modification highlights:
- 1.4mm lower height, gives 0.9mm margin above E3D nozzle
- 5mm wide robust connections to 40x40mm fans
- Circular cross section air nozzle, matching left-right ducts. This is going to blow air evenly, no matter which direction the head moves
- Circular fan mount