Lorenz attractor notebook

A quick notebook showing the lorenz attractor.

I'm not quite happy with this yet as I don't see how to explain the derivation of the particular variant of the Lorenz attractor that we're using (which has rho as a controllable parameters so you can move it into and out of the chaotic regime).  I'm pretty sure I just copied this from Chris at some point.  But it's good enough for showing some pretty pictures.