Add :serial and :meander unfolding schemes

[apkille]

These unfolding schemes complement the already implemented :interleaved and :fused schemes. I wrote them up for a project testing the performance of different quantics bases for multivariate functions in different settings. It's possible they could be of use to others. If the package maintainers disagree and want to close this PR, that's perfectly fine! If it gets merged I can follow up with a PR to QuanticsTCI.jl for support in the TCI interface.

[rittermarc]

Hi @apkille , and thank you for this pull request!

Unfortunately, it has a bit of an inopportune timing, since we're working on merging #21, which enables users of this library to choose the index order freely. The schemes :fused and :interleaved in the new implementation will just be shorthands to set up corresponding index orders, and we can certainly add your :serial and :meander keywords there.

If it's ok with you, I'll keep this pull request open for now. I'll work on merging #21 first, then implement your keywords, which have well-chosen names, and use your tests for the newer implementation.

[apkille]

Yep, that's OK with me! I will also submit the supporting PR for the unfolding schemes with QuanticsTCI.jl.

By the way, the packages in this GitHub org are very impressive and fast. Using them makes my life a whole lot easier :)

[shinaoka]

#21 has been merged into the main. How should we proceed?