Time-evolution MPO refactoring

[lkdvos]

In an attempt to reduce TTFX I refactored some of the code for the time-evolution MPO constructions, mostly just introducing a couple more function barriers.
Especially for the TaylorCluster implementation, the Val(N) dispatch should really help inference out quite a lot.
Additionally, this does actually affect runtime beyond TTFX, so overall this should be a good improvement. 🤞

[codecov]

Codecov Report

❌ Patch coverage is 97.32143% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/algorithms/timestep/taylorcluster.jl	96.15%	3 Missing ⚠️

Files with missing lines	Coverage Δ
src/algorithms/timestep/wii.jl	100.00% <100.00%> (+1.85%)	⬆️
src/algorithms/timestep/taylorcluster.jl	98.08% <96.15%> (-1.92%)	⬇️

... and 1 file with indirect coverage changes

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[VictorVanthilt]

I'm curious, did this significantly improve TTFX?

[lkdvos]

Less than I was hoping :( it did actually speed up the code for small hamiltonians by quite a bit, since all of the bookkeeping stuff and indexing stuff was using non-inferred CartesianIndex{N} where {N}, so at least there is that