Fix truncate! function

[jofrevalles]

Summary

This PR resolves a bug found in the truncate! function that erroed when maxdim kwarg was larger than the size of the bond that was about to truncate. In a similar way, it also erroed when the threshold was lower than the smaller element in the spectrum that was used to truncate.

Moreover, we fixed the mixed_canonize! function, since previously did not work as the truncate! function expected for MPS with MixedCanonical forms. We also fixed an unnecessary canonization step in the truncate! for Canonical forms.

Example

Without this PR:

julia> using Tenet; using Test; using LinearAlgebra

julia> gate = Quantum(reshape(LinearAlgebra.I(4), 2, 2, 2, 2), [site"2", site"3", site"2'", site"3'"])
Quantum (inputs=2, outputs=2)

julia> ψ = MPS([rand(2, 2), rand(2, 2, 2), rand(2, 2, 2), rand(2, 2)])
MPS (inputs=0, outputs=4)

julia> ψ = deepcopy(ψ)
MPS (inputs=0, outputs=4)

julia> canonize!(ψ)
MPS (inputs=0, outputs=4)

julia> evolved = evolve!(deepcopy(ψ), gate; maxdim=10)
spectrum = [7.129434737170108, 1.2106678577680343, 3.16516617082361e-16, 9.749816418484561e-18]
ERROR: BoundsError: attempt to access 2×2×4 Array{Float64, 3} at index [1:2, 1:2, 1:10]
Stacktrace: ....

julia> evolved = evolve!(deepcopy(ψ), gate; threshold=1e-100)
ERROR: MethodError: no method matching -(::Nothing, ::Int64)
The function `-` exists, but no method is defined for this combination of argument types.

Closest candidates are: ...

Everything now works, and we extended the tests to cover for this changes.

[starsfordummies]

nice!

[mofeing]

Everything perfect! Just one suggestion to remove .+ (0,0) because it does nothing.