Replace Nsymbol calls or add guards in fusion tree iteration

[borisdevos]

Deals with #514, at least partially.
Now we have

julia> Wis
(Vect[IsingBimodule]((1, 1, 0) => 1) ⊗ Vect[IsingBimodule]((1, 1, 0) => 1)) ← Vect[IsingBimodule]((2, 2, 0) => 1)

julia> blocksectors(Wis)
0-element Dictionaries.Indices{IsingBimodule}

which behaves the same way as

julia> Wu1
Rep[U₁](1 => 1) ← (Rep[U₁](0 => 1) ⊗ Rep[U₁](0 => 1))

julia> blocksectors(Wu1)
0-element Dictionaries.Indices{U1Irrep}

[lkdvos]

I do think this is probably quite a big performance hit right in the hot path of the fusiontree constructors though, I don't think there is typically a fast implementation of this. Additionally, I was kind of expecting to convert more things to Nsymbol, to actually make them error if the fusion is disallowed?

[borisdevos]

I'm misunderstanding then what kind of behavior we want. So we want Nsymbol checks there, such that forbidden fusion trees shouldn't ever be able to be looked at? And then at the tensor construction level, when colors mismatch this should throw an error, thus preventing constructing forbidden tensors?

[lkdvos]

I think that was what I was expecting, especially since we decided Nsymbol errors instead of returning 0

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/fusiontrees/iterator.jl	98.83% <100.00%> (+0.59%)	⬆️

... and 42 files with indirect coverage changes

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