Fix reindex! by returing the final index replace mapping

[jofrevalles]

Summary

This PR resolves #177 by correctly fixing the reindex of the site indices. To do that, we modified the replace! function by returning the final index mapping. I believe this is the most efficient way to do this since we could also get the mapping by comparing the tensor before and after the replace!, but this seems to do unnecessary work.

Example

Now the following code works fine:

julia> mps = rand(Chain, Open, State; n=5, p=2, χ=20)
MPS (inputs=0, outputs=5)

julia> mpo = rand(Chain, Open, Operator; n=5, χ=2, p=2)
MPO (inputs=5, outputs=5)

julia> inds(mpo, at = Site(1))
:A

julia> inds(mpo, at = Site(1; dual=true))
:F

julia> Tenet.@reindex! outputs(mps) => inputs(mpo)
MPO (inputs=5, outputs=5)

julia> inds(mpo, at = Site(1))
:A

julia> inds(mpo, at = Site(1; dual=true))
:O

julia> inds.(mpo)
5-element Vector{Vector{Symbol}}:
 [:K, :V, :B, :L]
 [:L, :U, :C, :M]
 [:M, :P, :D, :N]
 [:N, :J, :E]
 [:O, :A, :K]
 
julia> contract(merge(TensorNetwork(mps), TensorNetwork(mpo))) # We get a 5-dimensional tensor as expected
2×2×2×2×2 Tensor{Float64, 5, Array{Float64, 5}}: ...

[mofeing]

Suggested change

	@test issetequal([qtn2.sites[i] for i in inputs(qtn2)], [qtn.sites[i] for i in outputs(qtn)])
	@test issetequal([inds(qtn2, at=i) for i in inputs(qtn2)], [inds(qtn, at=i) for i in outputs(qtn)])

[mofeing]

Suggested change

	@test issetequal([qtn2.sites[i] for i in inputs(qtn2)], [qtn.sites[i] for i in outputs(qtn)])
	@test issetequal([inds(qtn2, at=i) for i in inputs(qtn2)], [inds(qtn, at=i) for i in outputs(qtn)])

[mofeing]

Suggested change

	@testset "complicated replacement" begin
	@testset "overlapping replacement" begin

[mofeing]

Use inds.(tensors(tn)) or another way to test this.