Document electronic transitions

[hartytp]

Currently we don't properly document how we define electric dipole / quadrupole matrix elements and how scattering rates / Rabi frequencies should be calculated from them.

This has been discussed in a few places, opening a single issue here to track the discussion:

• Epole matrix element #19
• Expose matrix element calculation #27
• Added physics documentation #10

One thing to think about here is what level of documentation we want. My aim is explicitly not to write a text book on atomic physics; just to have enough documentation for the conventions / definitions to be clear to someone with a good understanding of (text book on) atomic physics.

In particular, I want to avoid the kind of explanations that one often finds in atomic physics which go "detail...detail...detail...wave hands...detail...detail". I don't think that is a useful thing for us to have. As such, I think we probably just need a much condensed version of #10 , which summarizes the results, but without as much of the workings.

NB there are a few FIXMEs which have crept in in places where I think we're doing something slightly odd in our electric multipole calculations. We should resolve these while tidying up the docs here.