Multifusion compatibility

[borisdevos]

This PR adds some functions which are useful in multifusion settings. I don't know how often Base.oneunit(::SumSpace) is called within MPSKit, so no clue how much I'll actually be using this. I'm also just not sure how I should think of left/rightunitspace of a SumSpace, so I made one choice here that returns a 1-element space.

I kind of freestyled with coming up with tests, so feel free to criticize.

Edit: clearly I made the opposite choice in the tests, having confused myself at some point. So either the tests or the definition of left/rightunitspace need to change.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 54.72%. Comparing base (a98180a) to head (333a322).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #38      +/-   ##
==========================================
+ Coverage   54.56%   54.72%   +0.15%     
==========================================
  Files          17       17              
  Lines        1446     1451       +5     
==========================================
+ Hits          789      794       +5     
  Misses        657      657              

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[lkdvos]

Suggested change

	TensorKit.isunitspace(S::SumSpace{<:GradedSpace}) = all(isunitspace, S.spaces)
	TensorKit.isunitspace(S::SumSpace) = all(isunitspace, S.spaces; init=false)

This probably doesn't have to be restricted to GradedSpace, and I guess empty spaces are isomorphic to zero spaces so that should return false

[borisdevos]

Sadly, init isn't a valid keyword argument for all, so I added a janky !isempty check

[lkdvos]

I think given the rest of the functions, it probably makes sense to remain consistent and indeed return a length 1 SumSpace, although I'm not sure if that is the best option. Presumably it doesn't matter too much though

[lkdvos]

Can you also increase the patch version? I'll immediately tag a release