Compute S^z and S^2

[mrossinek]

Summary

After multiple fixes in Qiskit Nature, this PR now also adds support for computing S^z and S^2 in the PySCFGroundStateSolver.

Details and comments

Supporting the correct AngularMomentum with taking the alpha-beta overlap into account during the QiskitSolver is currently not possible because of the API workflow. In other words: the kernel method is not provided the information from which we can reconstruct this overlap matrix during the construction of the AngularMomentum property. We also cannot really compute it up-front and inject it into the QiskitSolver as an attribute, because this would require a-priori knowledge of the active MOs (which we do not have at that point).

This will need to be sorted together with the PySCF team, once I have raised the problem discussed here to them, since this affects their UCASCI and UCASSCF methods, too.

[coveralls]

Pull Request Test Coverage Report for Build 7084244416

• 26 of 26 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+2.6%) to 80.571%

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Totals
Change from base Build 6852319884:	2.6%
Covered Lines:	141
Relevant Lines:	175

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💛 - Coveralls