LDLFactorization subtype of LinearAlgebra.Factorization

[tmigot]

I think that LDLFactorization could be a subtype of LinearAlgebra.Factorization, cf. https://github.com/JuliaLang/julia/blob/master/stdlib/LinearAlgebra/src/factorization.jl .

We would benefit from the default Julia interface, and we would start some unifying of JSO factorization packages.