Split off norm-preserving vector retractions

[leburgel]

This splits of the core norm-preserving retraction utilities for vectors as vector_retract and vector_transport!, and updates the PEPS retractions to broadcast these across the unit cell. The motivation behind this is that I would like to reuse the core retraction tools when optimizing over generic vectors that support VectorInterface (e.g. a single TensorMap or a plain Vector{<:Number}) in settings where the state we're optimizing has additional structure and is therefore not a generic InfinitePEPS.

I also noticed that the utility/diffset.jl file was still included, but is no longer used anywhere. Since I think we anyway intended to move away from that, I thought it would be appropriate to just remove it.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/PEPSKit.jl	100.00% <ø> (ø)
src/algorithms/optimization/peps_optimization.jl	96.10% <100.00%> (-0.45%)	⬇️
src/utility/retractions.jl	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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[lkdvos]

It seems like somehow codecov is saying the retractions are actually never used. Is this true?

[EDIT] looks like the symmetrization code actually doesn't use the updated retraction, this should probably be fixed :)

[lkdvos]

Ok for me if tests turn green :)