Define Base.show for SUWeight

[Yue-Zhengyuan]

This PR defines Base.show for SUWeight, so we can use display(peps.weights) to see the diagonal elements in the weight matrices in an InfiniteWeightPEPS. It's a simple IO function so I don't have tests for it. A possible future improvement is to define a more compact output for println and print.

Example output for peps with U1Irrep symmetry:

SUWeight{TensorMap{Float64, GradedSpace{U1Irrep, TensorKit.SortedVectorDict{U1Irrep, Int64}}, 1, 1, Vector{Float64}}}
(1, 1, 1):
Irrep[U₁](0) = [1.0, 0.013284183529494106, 0.01025508746347934]
Irrep[U₁](-1) = [0.155297993782553]
(2, 1, 1):
Irrep[U₁](1/2) = [1.0, 0.013284183529494106]
Irrep[U₁](-1/2) = [0.155297993782553]
Irrep[U₁](3/2) = [0.01025508746347934]
(1, 2, 1):
Irrep[U₁](0) = [1.0, 0.013284183529494106]
Irrep[U₁](1) = [0.155297993782553]
Irrep[U₁](-1) = [0.01025508746347934]
(2, 2, 1):
Irrep[U₁](0) = [1.0, 0.013284183529494106, 0.01025508746347934]
Irrep[U₁](1) = [0.155297993782553]
(1, 1, 2):
Irrep[U₁](0) = [1.0, 0.013284183529494106]
Irrep[U₁](1) = [0.155297993782553]
Irrep[U₁](-1) = [0.01025508746347934]
(2, 1, 2):
Irrep[U₁](0) = [1.0, 0.013284183529494106, 0.01025508746347934]
Irrep[U₁](1) = [0.155297993782553]
(1, 2, 2):
Irrep[U₁](0) = [1.0, 0.013284183529494106, 0.01025508746347934]
Irrep[U₁](-1) = [0.155297993782553]
(2, 2, 2):
Irrep[U₁](1/2) = [1.0, 0.013284183529494106]
Irrep[U₁](-1/2) = [0.155297993782553]
Irrep[U₁](3/2) = [0.01025508746347934]

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 9 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/states/infiniteweightpeps.jl	0.00%	9 Missing ⚠️

Files with missing lines	Coverage Δ
src/states/infiniteweightpeps.jl	81.81% <0.00%> (-8.19%)	⬇️

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[pbrehmer]

Looks good to me, I'd be fine with merging this! Some tests sadly still time out but that's unrelated to this PR, see #166. We'll have to figure that out separately.