Use Styles interface for expectation values

[AFeuerpfeil]

In this PR, I implement the GeometryStyle and OperatorStyle interface for expectation_value.
The main entry points are:

function expectation_value(ψ::AbstractMPS, H::AbstractMPO)
    return expectation_value(GeometryStyle(ψ, H), OperatorStyle(H), ψ, H)
end
function expectation_value(
        ψ::AbstractMPS, H::AbstractMPO,
        envs::AbstractMPSEnvironments
    )
    return expectation_value(GeometryStyle(ψ, H), OperatorStyle(H), ψ, H, envs)
end

@lkdvos: Happy to take feedback from you for this small PR! When we have converged on a good convention, I will spend the next days to finish up the styles rewrite.

[github-actions]

Your PR no longer requires formatting changes. Thank you for your contribution!

[lkdvos]

That's great news, happy you are back, and happy to discuss :)