fix correlation length unit cell

[ogauthe]

Fix #197

The current fix also changes the definition of the correlation length: it includes the size of the unit cell inside. I am not sure about this change, I can discard it if it breaks MPSKit conventions.

I still think there is something wrong: in the AKLT case, this fix gives a correlation length around 1 when I believe it is around 2 (as published in https://link.aps.org/doi/10.1103/PhysRevB.96.121115), but I am less sure where the factor 2 is wrong.

I can add the AKLT case as a test, regardless of the factor 2 just to check if it executes.

[lkdvos]

Thanks for the quick fix. Any chance you could add a test to that would break before and no longer does now? No worries if not, it’s just better in the long run :)

[ogauthe]

This is only a fix for the unit cell coordinate. As mentioned in #199 , there is another issue with this function that requires further work.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/algorithms/toolbox.jl	98.07% <100.00%> (-0.08%)	⬇️

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