Add iPEPS SUWeight to CTMRGEnv conversion

[Yue-Zhengyuan]

Motivated by #264, this PR adds the conversion of the SUWeight environment of an iPEPS to a product-state CTMRGEnv. The no-so-elegant syntax is

CTMRGEnv(wts::SUWeight, peps::InfinitePEPS)

The reason for needing peps is that wts is stored in such a way that the axis order of each weight is dependent on the arrow direction in the PEPS (see discussions in #239). E.g. for x-weights, it is either 1 ← x ← 2 or 2 → x → 1.

The conversion is tested by comparing the 1 x 1 reduced density matrices obtained by a manual construction and reduced_densitymatrix.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/environments/suweight.jl	87.23% <100.00%> (+3.02%)	⬆️

... and 1 file with indirect coverage changes

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[leburgel]

This looks great, and would be really good functionality to have.

It would be great to revisit #223 at some point and try to merge common functionality between the BPEnv there and the environment being constructed here. But at this point, I think it's good to have this as is.