Bump v0.6.1

[lkdvos]

Since it has been a while since we had a new release, and we did add a bunch of optimizations and improvements, I would suggest registering a new patch release before we do any further changes.

Release notes:

• This version contains a number of performance improvements, specifically for the dominant contractions within CTMRG for PEPS.
• We now support constructing reduced density matrices (reduced_densitymatrix), which also has a more efficient implementation than before. This is now used for obtaining expectation values as well.
• There now is built-in support for computing correlators along the horizontal or vertical directions (correlator).
• There is a new truncation mode (SiteDependentTruncation) to allow specifying site-dependent truncation schemes
• Several small internal changes were made to improve type stability.

[codecov]

Codecov Report

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[Yue-Zhengyuan]

I suggest to be more specific on where the new features are in the release notes.

• We now support constructing reduced density matrices (reduced_densitymatrix), which also have a more efficient implementation than before, and is now used for obtaining expectation values as well.
• There now is built-in support for computing correlators along the horizontal or vertical directions (correlator).
• There is a new truncation mode (SiteDependentTruncation) to allow specifying site-dependent truncation schemes.

I'm still a bit uncomfortable about normalization in SU in this version, and it is affecting some of my works. Other than this, 0.6.1 is OK for me.

[leburgel]

Looks good to me, and also agree that being more specific on where the new features are exactly could be helpful.

However, it's not really ideal to have a patch release missing from the docs since the build failed here. Any chance we could fix that before tagging?

[lkdvos]

Linking this here: JuliaDocs/DocumenterInterLinks.jl#18

[lkdvos]

Ok, should be good to go once tests pass, and one person approves the PR. Feel free to immediately tag a release as well, if not I will probably do this tomorrow.