Refactor of IDMRG with IterativeSolver

[AFeuerpfeil]

In this PR, I refactor IDMRG to follow the IterativeSolver interface (as VUMPS).
This allows to share more code between IDMRG and IDMRG2 and also simplifies overloading some parts of the algorithm for e.g. different MPO types.

[github-actions]

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/algorithms/groundstate/idmrg.jl	100.00% <100.00%> (+1.44%)	⬆️
src/utility/iterativesolvers.jl	88.88% <100.00%> (ø)

... and 17 files with indirect coverage changes

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[AFeuerpfeil]

@lkdvos: I have implemented the points we discussed today.
One slight issue I had with making the energy typestable is that if mps or mpo are complex, the energy obtained from fixedpoint is real, iff ishermitian. Therefore, I added the constructor IDMRGState{T}. An alternative solution would be to directly call

it.state = IDMRGState(mps, state.operator, envs, state.iter + 1, ϵ, T(E_new))

I am not sure, which way I prefer.

[AFeuerpfeil]

PS: The one-line change in IterativeSolver is the bugfix that was already merged some time ago.

[lkdvos]

Overall looks great, thank you!