Relativity errata

I of course screwed up and left out the explanation of Lorentz Contraction from the obvious place in Part 3 where I should have talked about it.

So I've slotted it in (plus a new diagram to make super-clear what's happening).

Or if you want to get the flow/parallelism there's now a link to the earlier time dilation discussion (which I also cleaned up a bit), since that's really the same issue but turned on its side 90°.

so maybe the Stargate SG-1 example will be clearer now. or maybe not.