Wrap SU intermediate steps into functions

[Yue-Zhengyuan]

This PR refactors the simple update, so that some parts can be reused by full update / neighborhood tensor update in the future.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/algorithms/time_evolution/evoltools.jl	100.00% <100.00%> (ø)
src/algorithms/time_evolution/simpleupdate.jl	87.01% <100.00%> (+0.10%)	⬆️

[lkdvos]

Overall looks good to me.
Something to consider in the future (separate PR) is the same as before: have you considered whether the "linear map" choices are the most convenient?
It seems like permuting everything to TensorMap{N,0} might lead to unnecessary permutations again.

[Yue-Zhengyuan]

have you considered whether the "linear map" choices are the most convenient?

I'll leave this PR open for a while and see if I can further optimize it. I already kind of lost track of it, as you can see that I didn't specify the number of codomain/domain indices for the reduced tensors a, b this time...

[Yue-Zhengyuan]

@lkdvos Or, is it ok if I just merge this one as it is and consider the choice of codomain/domain in the next PR on the construction of bond environment? That can provide more context on which choice is the best

[lkdvos]

I actually prefer it in separate PRs, as you can tell the small ones are a lot easier to review and get progress on :).

Ok to merge for me.