[Dune] calling ISTL solvers for dense matrices

[Christian Engwer]

> Yes, we definitely need a fallback to a default allocator for those cases, either
> with SFINAE trickery or by just having a freestanding AllocationTraits struct
> that can be specialized for containers that export an allocator.

Some kind of allocator traits is perhaps the best option. This would
even allow specialize for a particular combination of FieldVector and
field type.

I'll go for that option.

Christian