Expose semi-stable API for excitation (eigenmode) viewer

[kbarros]

A prototype eigenmode viewer is available in SunnyContributed: https://github.com/SunnySuite/SunnyContributed/blob/main/initiatives/Sam-GT/eigenmodes/eigenmode_viewer.jl

It currently depends on some Sunny internals that are subject to change. For example, extracting the net magnetic moment from an SU(N) coherent state will depend on information that is stored inside System. We should investigate whether there is some simple API that could be moved into core Sunny such that eigenmode_viewer.jl can be protected from breaking changes. I am not sure what that API will look like, or if it needs to be exported into the user-facing API. Let's revisit this issue after the entangled units development work has matured.