Branching ratios between hyperfine and fine basis

[dihm]

I had a problem recently that needed to solve a system with sublevels from a fine Rydberg state and a low-lying hyperfine state. ARC provides all the tools, but I found what I think are gaps in calculating transition rates.

This PR adds some companion functions to the existing getBranchingRatio (which only covers hyperfine->hyperfine transition rates). Ultimately these functions are simple enough that there is an argument that they aren't necessary. But I think it would be good to add them to ARC since working them out has some tricky aspects for the average ARC user:

• the FS to FS one uses the currently undocumented getSphericalDipoleMatrixElement method, so replicating functionality requires reading source code
• going between HFS and FS requires scaling factors that I would call non-obvious. For those who aren't experts in reducing matrix elements (like myself), it would be nice to have a vetted source of truth.

Speaking of vetted source of truth, it would be good for others to confirm this experimentalist has actually calculated things correctly. I also have no preference regarding syntax or coding style, I merely tried to follow conventions set by getBranchingRatio. Happy to make whatever changes deemed necessary.

[nikolasibalic]

I would like to understand (2 * self.I + 1) factor in FS to HFS better than I do at the moment. As far as I can tell, comparing with different calculations, this should be ok. I will merge this, and if there are any remaining errors in this section, we will figure out with use.