Add a new SAOOVQE solver

[MartinBeseda]

I'm writing here with a question, as I'd like to submit a contribution to Qiskit Nature in the form of a new State-Averaged Orbital-Optimized VQE, containing also some new approach to orbital diabatization etc. But, as it is quite a large code, I didn't want to open a PR right away, as I'd like to ask, how should I proceed...

If I do understand the Nature's structure correctly, should we start with including it into second_q/algorithms/excited_states_solvers?

The repository of the whole code is here: https://gitlab.com/MartinBeseda/sa-oo-vqe-qiskit
And the documentation is online here: https://sa-oo-vqe-qiskit.readthedocs.io/en/latest/index.html

P.s.: Yes, I've read the contributing guidelines, we'll supplement CLAs, etc. right with the submission.

Thank you very much for all the opinions and advice!

[MartinBeseda]

@mrossinek Do you think it could be of interest to add our solver? Thank you for the info!