Finite-T improvements and utility

[lkdvos]

This PR contains a bunch of features and fixes that are (somewhat) related to finite temperature simulations.

• I fixed a δ output not being passed on in approximate calls
• I improved support for real and complex valued tensors, and mixtures. In particular, by indicating imaginary time evolution explicitly with a flag, it is possible to use real numbers in time-evolution, which was previously not possible.
• I added various contractions that were missing for working with density matrices.

[codecov]

Codecov Report

❌ Patch coverage is 89.78102% with 14 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/operators/mpo.jl	25.00%	6 Missing ⚠️
src/states/infinitemps.jl	71.42%	4 Missing ⚠️
src/algorithms/timestep/tdvp.jl	83.33%	3 Missing ⚠️
src/operators/mpohamiltonian.jl	83.33%	1 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/approximate/approximate.jl	100.00% <100.00%> (ø)
src/algorithms/changebonds/svdcut.jl	94.11% <100.00%> (ø)
src/algorithms/expval.jl	88.60% <100.00%> (+2.67%)	⬆️
src/algorithms/timestep/integrators.jl	75.00% <100.00%> (+3.57%)	⬆️
src/algorithms/timestep/taylorcluster.jl	100.00% <100.00%> (ø)
src/algorithms/timestep/time_evolve.jl	100.00% <100.00%> (ø)
src/algorithms/timestep/wii.jl	98.14% <100.00%> (+35.18%)	⬆️
src/operators/jordanmpotensor.jl	82.94% <100.00%> (+1.35%)	⬆️
src/states/finitemps.jl	82.49% <100.00%> (+0.11%)	⬆️
src/states/orthoview.jl	89.68% <100.00%> (+0.33%)	⬆️
... and 5 more

... and 1 file with indirect coverage changes

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[VictorVanthilt]

This is a very welcome addition!!!