[dune-fem] Generalize Dune:Fem::CombinedDiscreteFunctionSpace to an arbitrary number of spaces

[Agnese, Marco]

Hi everybody,
I think it would be very useful to generalize the Dune::Fem::CombinedDiscreteFunctionSpace in order to accept an arbitrary number of spaces. 
It would be much more user-friendly.

I saw that Tobias Malkmus is working on the combined space in these days and it seems that he is generalizing some classes using variadic templates. Am I wrong? 
Do you project to use also variadic templates for Dune:Fem::CombinedDiscreteFunctionSpace in order to accept an arbitrary number of spaces?

Thank you very much,
best reagards,
Marco.