Make a few things thicker, add space between parts
modularize, play with thingiverse customizer stuff
Add a small raft for better printing
Move magnet cavity

This moves the attachment magnet cavity to merge it with the train
magnet.  This makes for better structure and once glued in, the
inner magnet should be just as secure as if it was in its own cavity
(possibly more since I could never get the wall between the two
cavities quite right).
Make it 1mm taller
Latest print test.

- Remove old magnet-cavity code; I'm not going to use it
- Tweak height and support material settings
- Adjust mounting-magnet radius value based on mounting block dimension
- Create separate radius value for inner magnet to give a tiny bit of additional leeway