MasterCore spool core screw-on

This is a remix of Callum Coles' original components for [3DTomorrow's](https://3dtomorrow.com/) MasterCore.

I found that the flanges had a habit of falling off under stress (like when I pick a half-empty spool up and squeeze them together slightly) and spaghetti-like catastrophe ensues!

So I added a screw-thread to the "faces" and created a nut.

The MasterCore concept is demonstrated in Callum's video - https://youtu.be/kI3uXl-MNO4

The "arms" are Callum's original, unsullied component.

Only the nut needs support. I found it best to print it with the flange-side uppermost so as to make the start of the screw thread cleaner.