Generic array types

[gdalle]

Here is a summary of some of the issues encounted with generic array types:

• Krylov.jl solvers want to work with vectors and not arrays, so we reshape everything first
• DifferentiationInterface.jl v0.7 imposes that preparation types equal execution types (unless we relax the "strict" option)
• LinearOperators.jl wants V(undef, n) to be defined for any vector type V, which errors e.g. for V = ComponentVector so in general it is easier to set V = Vector
• Pre-allocating a space with the correct vector type and doing copyto!(::CustomType, ::Vector) before the pullback doesn't work when CustomType is not writable (e.g. reshape of UpperTriangular)

Related:

• Compatibility with ComponentArrays #152
• CompatHelper: bump compat for ImplicitDifferentiation in [weakdeps] to 0.8 for package ControlSystemsBase, (keep existing compat) JuliaControl/ControlSystems.jl#998
• Support ComponentArrays JuliaSmoothOptimizers/Krylov.jl#701
• Allow more generic storage type S JuliaSmoothOptimizers/Krylov.jl#705
• Support generic arrays JuliaSmoothOptimizers/Krylov.jl#706