[dune-fem] Possible bug when defining a linear operator between spaces whihc live on different grid

[Agnese, Marco]

I answer to my question.
There is no bug. 

The error is that I have used a Dune::Fem::Stencil without calling the method fill(). 

If I use a Dune::Fem::SimpleStencil it works correctly.

Cheers,
Marco.