Adapt simple update to DiagonalTensorMap

[Yue-Zhengyuan]

Changes:

• Restricts some structs and functions about the singular value spectrum (most notably SUWeight and sdiag_pow) to work with DiagonalTensorMaps.
• Some refactoring of simple update.
• Added an example of Heisenberg model simple update. It will later be expanded with examples on NTU and full update.

[codecov]

Codecov Report

Attention: Patch coverage is 82.50000% with 7 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/utility/util.jl	76.47%	4 Missing ⚠️
src/algorithms/ctmrg/sparse_environments.jl	0.00%	2 Missing ⚠️
src/algorithms/time_evolution/simpleupdate.jl	91.66%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/PEPSKit.jl	87.50% <ø> (ø)
src/algorithms/time_evolution/evoltools.jl	100.00% <100.00%> (ø)
src/states/infiniteweightpeps.jl	93.50% <100.00%> (+0.17%)	⬆️
src/algorithms/time_evolution/simpleupdate.jl	86.90% <91.66%> (-1.01%)	⬇️
src/algorithms/ctmrg/sparse_environments.jl	29.62% <0.00%> (ø)
src/utility/util.jl	54.00% <76.47%> (+1.36%)	⬆️

[Yue-Zhengyuan]

The failed tasks involve the backprop of sdiag_pow which now only works with DiagonalTensorMap, although the forward pass is fine. I may need help on fixing this since I'm not familiar with Zygote, ChainRules etc...

[lkdvos]

The rrule you have looks fine, the tests are just complaining because you removed the implementations of these functions for the TensorMap case, which I don't think is necessary.

(In any case we still have to support TensorMap for now since the projector onto the diagonal type is currently not working properly, and we might lose that information. See also QuantumKitHub/TensorKit.jl#208.

[lkdvos]

Overall, looks good to me! In principle, the examples could do with some clean up, but maybe we can leave that for when we uniformize the documentation.
(Also, if possible in the future to split of different topics into different PRs that would be great).

After updating with the master and my final comment this should be good to go for me.

[pbrehmer]

Thanks for the work @Yue-Zhengyuan, this looks good! As long as we make sure to credit the YASTN code bits and adapt the naming to our internal conventions in the follow-up PR, I'm happy to merge this :)