Reorganize CTMRG contraction code

[Yue-Zhengyuan]

This PR splits src/algorithms/contractions/ctmrg_contractions.jl into several shorter files in the folder src/algorithms/contractions/ctmrg/ for easier navigation among different parts.

No changes are made in docstrings or code body, so the PR can be safely merged as long as the tests pass.

[codecov]

Codecov Report

❌ Patch coverage is 50.89820% with 246 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/contractions/ctmrg/fullinf_env.jl	2.89%	67 Missing ⚠️
...lgorithms/contractions/ctmrg/renormalize_corner.jl	43.85%	64 Missing ⚠️
src/algorithms/contractions/ctmrg/halfinf_env.jl	6.81%	41 Missing ⚠️
.../algorithms/contractions/ctmrg/renormalize_edge.jl	65.88%	29 Missing ⚠️
...rc/algorithms/contractions/ctmrg/enlarge_corner.jl	65.00%	28 Missing ⚠️
src/algorithms/contractions/ctmrg/projector.jl	38.46%	8 Missing ⚠️
src/algorithms/contractions/ctmrg/expr.jl	82.05%	7 Missing ⚠️
src/algorithms/contractions/ctmrg/gaugefix.jl	95.45%	2 Missing ⚠️

Files with missing lines	Coverage Δ
src/PEPSKit.jl	100.00% <ø> (ø)
src/algorithms/contractions/ctmrg/contract_site.jl	100.00% <100.00%> (ø)
src/algorithms/contractions/ctmrg/gaugefix.jl	95.45% <95.45%> (ø)
src/algorithms/contractions/ctmrg/expr.jl	82.05% <82.05%> (ø)
src/algorithms/contractions/ctmrg/projector.jl	38.46% <38.46%> (ø)
...rc/algorithms/contractions/ctmrg/enlarge_corner.jl	65.00% <65.00%> (ø)
.../algorithms/contractions/ctmrg/renormalize_edge.jl	65.88% <65.88%> (ø)
src/algorithms/contractions/ctmrg/halfinf_env.jl	6.81% <6.81%> (ø)
...lgorithms/contractions/ctmrg/renormalize_corner.jl	43.85% <43.85%> (ø)
src/algorithms/contractions/ctmrg/fullinf_env.jl	2.89% <2.89%> (ø)

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[Yue-Zhengyuan]

I still have some reordering to push, but I'll wait until the current test finishes.

[Yue-Zhengyuan]

Bad news: we can't just merge this PR due to the poor test coverage (50%).

It appears to be mainly due to not testing CTMRG for PEPS-PEPO-PEPS sandwich. The very low coverage of fullinf_inv and halfinf_inv (< 10%) is also surprising to me (maybe due to not testing IterSVD?).

@pbrehmer We may need your help to increase test coverage, or identify and remove things that are not used.

[pbrehmer]

The low test coverage of halfinf_env and fullinf_env indeed stems from the fact that we don't have a sparse CTMRG implementation yet. By sparse I mean that we never explicitly construct the environment and then use a sparse SVD with a function handle. Still, I added these sparse contractions at some point because I had them lying around or I was trying out something (can't remember) and I decided to leave them in for future use. All the code that is actually in use should be tested if I remember correctly.

So at least with respect to the half/full-infinite environment things, the low test coverage isn't saying much. We could easily increase it by just commenting out the unused parts. I could do that if we want to do it. (Not sure about the PEPS-PEPO-PEPS things though, I assumed there was tests for that...)

[lkdvos]

In any case, I would suggest to not do any of that in this PR, I really like that this is only reorganizing the files.

Overall I think this is a great change, let's wait for the tests to run just to be sure but since we aren't actually changing any of the test coverage I would just merge afterwards.