[dune-functions] Interface limitations in the functions interface

[Christian Engwer]

OK, so some further insights...

> >A)I have the impression that it should be not to complicated
> >  to implement this by extending the current interface with
> >  varidadic templates. But I'll have to look into the details.
> >  On thing that I'd really like to avoid is supporting a
> >  total derivative wrt all domains at one then, because it
> >  will in general be hard to define a proper range type for this.
> 
> I started a prototype implementation for this... it is feasible, although not totally straight forward

it is owrking now, but only for the derivatives, I didn't yet look
into anything like localView, or bind.

I seems to be not too complicate :-) if you want, you can have a look
at the feature/multi-argument-functions branch.

Besides these experiments, I think we can work around the problems in
dune-pdelab, at least if we don't use the virtual interface. For the
virtual interface, it would be good to have a hook in the interface
wrapper, which allows to inject other interfaces. I have an idea about
how to do this, but I didn't test it yet. This owuld allow us to reuse
major parts of the code, derive our own typeerasure class from
DifferentiableFunction and be able to inject the setTime method into
this wrapper.

Ciao
Christian