Add 'curly braket' to indicate hyperfine splitting

[tmonz]

Background/Motivation: consider an atomic ground state with hyperfine splitting. The splitting is commonly depicted with a curly barket between the levels, and the energy difference between the two stated next to the curly braket e.g. in GHz. It's not clear to me how to do it in the current library.

Goal: Find a way to add curly brakets such that spliting or small differences in energy can be denoted next to a curly braket between two levels.

Expected outcome: Add functionality that (likely) takes the two leves, placement of the curly braket, and a statement for the energy difference.

[tmonz]

Something along those lines

[dihm]

Thank you for the suggested enhancement (and sending an example figure)!

I'll admit that this is not a convention that I typically use and so I don't know when (or if) I would add this as a direct feature to the library. That said, LD provides the matplotlib figure/axes handles. One could simply add their own implementation on top of it to add this feature. Frankly, such a prototype is necessary to fully incorporate into the library anyway, so creating it would be very helpful to that end.

At first blush, I would imagine the steps to produce this would be:

1. Use the graph specification for the levels to determine the location on the axes for the edge of the two levels to be coupled
2. Place a text object of a curly bracket with the appropriate size and offset to "couple" the levels
3. Place a second text object with the label at the correct offsets from the bracket

I'll note that use of a text bracket could be problematic since the exact sizing and placement will be font dependent. Defining a functional form that plots it as a line would be better (much like how the wavy arrows are currently handled), but it is not immediately obvious to me what that function would look like. Though a quick google search shows someone has worked on this already.

Of course, if you need something more immediately, you can also go back to manual mode and export the LD plot as SVG then import to inkscape (or similar) and add this annotation there. A very unsatisfying answer, but needs must when things need getting done.