Backport Dune::Fufem::LaplaceAssembler from the 2.9 branch
This is the only thing I need to still build and run my complete dune-gfe module (the implementation of the geometric finite element method) against the 2.8 release of "everything".
The patch only adds code in the namespace Dune::Fufem. It should therefore not influence the behavior of any code currently using dune-fufem 2.8.