Spinners for MMU2

Here are some spinners for the MMU2/S unit.

You need two 8x3 magnets for every spinner.

I have included a multimaterial amf file with a complete set of spinners.

Enjoy!

**Updates:**
---------------

2020/01/03:

+ Improved Arrow_Spinner design (multimaterial .3mf file).

2020/02/22:

+ Added a new spinner (use two 7mm magnet)

**2020/03/05:**

+ Added a new spinner (use two 8mm magnet)