Discovery One User Experience Improvements

This is a series of improvements for metaform3d's amazing Discovery One model.

The only problems I had with that model is that many of the parts have to be glued together in concert, in a way that only fits when all the parts are together. A few tabs and slots would make construction easier.

The main Core modules along the spine now have a tab and slot to align these pieces easier. The Rear sections now can fit to the brackets without sliding, so you can put those together before adding th