Docs additions related to constructing a given symmetric TensorMap

[leburgel]

Some docs additions that should make it easier for users to find out how to construct symmetric TensorMaps corresponding to specific physical systems. In summary I:

• Explicitly documented the fields of the FusionTree type, since these need to be accessed explicitly when assigning data slices labeled by splitting-fusion-tree pairs.
• Documented the fusiontrees iterator generator, both in lib/sectors and lib/tensors.
• Explicitly documented the fields of all exported Sector subtypes, since again these need to be accessed explicitly when assigning data slices.
• Added docs for FermionNumber and FermionSpin.
• Added docstrings for the TensorMap constructors to lib/tensors.
• Documented the different flavors of getindex and setindex! for TensorMaps in lib/tensors.
• Expanded the section on "tensor maps from existing data" in man/tensors to be a bit more explicit about the Heisenberg example.
• Added a section in man/tensors on how to assign data slices after initializing a TensorMap, both by using block(::TensorMap, ::Sector) to assign the full matrix blocks, and using the setindex! syntax t[f₁::FusionTree, f₂::FusionTree] = v to assign slices of t labeled by splitting-fusion-tree pairs (f₁, f₂).

The changes to man/tensors might make it a bit physics-oriented, but since this example was already being used I didn't see much harm in making it more explicit like this. I anyway wouldn't know how to explain the splitting-fusion-tree slice assignment in any other way.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.40%. Comparing base (ce96d55) to head (47ce41d).

Additional details and impacted files

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  Coverage   81.40%   81.40%           
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  Files          42       42           
  Lines        5565     5565           
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  Hits         4530     4530           
  Misses       1035     1035           

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[Jutho]

Thanks; this looks great. I've added only a few minor comments.

[lkdvos]

I think everything is addressed now, shall I merge?

[Jutho]

Yes, I had planned to do this yesterday but wanted to check with Lander whether this was ready and then somehow forgot.