Round hub springs with endstops for autorewind spool holder

The spool holder tubes that came with my MMU2S are just a bit too short for the rewind spool holder stand.

This remix just adds a small 4.5mm stop to the springs so the tube fits the stand and PTFE holder.

Also, (more important?) keeps the tube from always falling out on the spring side.

7/21/19: Added versions with knobs. These make it easier to position the hub in the spool holder stand, and also allow you to pre-tension the spring without having to rotate the spool.

Note: the firs