[lkdvos]

Builds on #40.

This PR adds eachstep(problem, algorithm; kwargs...), which initializes the state and returns an iterator that performs one step per iteration and yields the state.
This allows running arbitrary code in between steps, without having to write a new algorithm or logging action:

iterator = eachstep(problem, algorithm; iterate = x0)
for state in iterator
    state.iteration % 10 == 0 && save_checkpoint(state.iterate)
end
result = finalize_state!(iterator)

• The loop body runs before the next stopping check, so it may modify the state.
• Breaking out of the loop keeps the iterator resumable, and finalize_state!(iterator) emits :Stop if needed and returns the result.
• initialize_state!(iterator; kwargs...) prepares the iterator for another run.
• solve and solve! are now implemented through this iterator.
• solve_loop! is deprecated. Since solve no longer calls it, methods added to it have no effect; a custom main loop is now provided by specializing Base.iterate on the iterator.

The docs have a new section on driving the loop yourself.

[codecov]

Codecov Report

❌ Patch coverage is 92.15686% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/interface/iterator.jl	88.57%	4 Missing ⚠️

Files with missing lines	Coverage Δ
src/AlgorithmsInterface.jl	100.00% <ø> (ø)
src/deprecated.jl	100.00% <100.00%> (ø)
src/interface/interface.jl	100.00% <100.00%> (ø)
src/interface/iterator.jl	88.57% <88.57%> (ø)

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