In contrast to L2FunctionalAssembler this uses
the new dune-functions grid function interface.

In principle I would like to use one assembler
that works with both interfaces. I'm working
on a grid function wrapper mapping the old
dune-fufem interface to the new dune-functions
interface. Once this is working and tested we
can replace the old L2FunctionalAssembler by
the new DuneFunctionsL2FunctionalAssembler
using the wrapper if necessary.