Prusament spool to master spool adapter

This is an adapter that allows you to convert an (empty) prusament plastic spool to a master spool.

Motivation
====================
I love the concept of the [master spool](https://www.thingiverse.com/thing:2769823), but it's quite a heavy print (6+ hours for part B, 12+ hours for Part A). I saw that the [prusament](https://prusament.com/) spools could dimensionally fit and be re-used as master spools with a simple adapter. I iterated over several designs and came up with this 3 part design, where the adapter core (big cylinder) is required and the screw and nut are optional (you can also use zip ties to fix the two parts of the spool together in the middle instead of the screw and nut provided here). This makes it a much faster and cheaper solution than printing the master spool, and it also helps reusing a plastic part that would otherwise (possible) go to waste.

Prusament spools have a cardboard core, and you can even re-use that to create something nice such as [this](htt