Soverfactory throws error for --linear-solver=amg

[blattms]

because it is not registered, but our docs say that this is possible,

[atgeirr]

This requires --matrix-add-well-contributions=true (i.e. that the operator is a matrix operator). Maybe we could relax that requirement, or at least output a more helpful message. This applies to both serial and parallel runs.

The code in the factory:

        if constexpr (std::is_same_v<O, Dune::MatrixAdapter<M, V, V>>) {
            F::addCreator("amg", [](const O& op, const P& prm, const std::function<V()>&, std::size_t) {
                ...