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 the top and bottom. The nuts for the Core now fit into the new slots. This should help construction.

So I updated a few parts in order to make construction easier, and does not hinder the easy-printing of this model.