Specify eltype for CU1Irrep product iterator

[lkdvos]

I am not sure if this actually has some performance implications in the code, but at the very least it changes the following:
Before:

julia> collect(CU1Irrep(0) ⊗ CU1Irrep(1))
1-element Vector{Any}:
 Irrep[CU₁](1, 2)

---

After:

julia> collect(CU1Irrep(0) ⊗ CU1Irrep(1))
1-element Vector{CU1Irrep}:
 (1, 2)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.40%. Comparing base (fe3c9c2) to head (dc1d33c).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #115      +/-   ##
==========================================
+ Coverage   81.29%   81.40%   +0.11%     
==========================================
  Files          42       42              
  Lines        5575     5576       +1     
==========================================
+ Hits         4532     4539       +7     
+ Misses       1043     1037       -6     

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[Jutho]

Maybe there can be some extra tests in test/sectors.jl:
e.g. below line 21

@test eltype(it) == I
@constinferred collect(it)

[lkdvos]

I was looking for something to test, but collect(it) does not actually help, the inferred type is Vector{Any} and this is what it returns, so no error is raised. Similarly I got stuck with trying to test iterate(it), this is also correctly inferred, it is just the iterator interface of julia Base that is a bit strange, as it defaults eltype to Any.
Testing the eltype directly is definitely better, I really should have thought of that 😁

[Jutho]

Maybe typeof(collect(it)) == Vector{I} as extra test?

[Jutho]

Looks great; thanks. I'll merge when the tests are complete.