[dune-functions] Interface limitations in the functions interface

[Christian Engwer]

> I also wanted to do this for GridFunction and LocalFunction, but
> did not find a working solution. But I can't remember the reason
> now. Making the base class a template parameter of the wrapper
> should work.

That was kind of my idea. I thought that the type-erasure class could
get a pair of classes, which should be injected as base classes for
the interface and for the wrapper. These classes would have to be
template template parameters, as we would still have to specify the
main interface class, in order to inject the lower-level interfaces,
or the other way around...

Christian