fix: ignore input type in LinearOperator construction by default

[gdalle]

• By default, create the LinearOperator associated with A without providing S = typeof(input). Specifying S requires S(undef, n) to be defined, which is false for many custom vector types. Instead, we keep the default fallback on S=Vector, which is more likely to work.
• Add a keep_input_type keyword argument to OperatorRepresentation in order to allow the old behavior if necessary, e.g. on GPU. This does nothing for FunctionMap.
• Add a posdef argument to OperatorRepresentation, which FunctionMap can take advantage of. This does nothing for LinearOperator.
• Specify that the kwargs of IterativeLinearSolver{:Krylov} might be passed to gmres or block_gmres

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/execution.jl	100.00% <ø> (ø)
src/settings.jl	100.00% <100.00%> (ø)

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