Diffraction normalization should be S(k=0) = N

[bdice]

We should change the normalization criteria of the DiffractionPattern such that S(k=0) = N. Currently it is S(k=0) = 1. The typical formulation in physics (see Wikipedia) gives a result of N but we changed it to 1 so it's easier to plot. However, we can just adapt the plotting code to re-scale as needed, and keep the normal physics convention.

Originally posted by @bdice in #777 (comment)