Expressing operators with respect to their spin symmetry

[lelemmen]

Describe the feature you'd like
The current RSQOneElectronOperator is designed to be used in singlet restricted calculations, as are RSQTwoElectronOperator and RSQHamiltonian.

• Can we express other types of operators, like S_x, S_y and S_z in an unrestricted and restricted formalism?
• Can we express S+ and S- in restricted and unrestricted bases?
• Can we express spin triplet operators, or even more generally, spin tensor operators in our code in an intuitive way?

If we figure out how to express these kinds of operators in a restricted or unrestricted formalism, we further strengthen the connection of our code with the actual theory, and we might then have a nice way of representing spin-orbit operators, like the effective spin-orbit interaction operator that is discussed in Helgaker in chapter 2.2.3.

'General' (G) calculations/classes are the most general type, encompassing all other types, so we can then check if the (spin) operators are correctly implemented by checking their implementation with those in a GSpinorBasis.