Switch function interface in `GeneralizedLaplaceAssembler`

* Switch to dune-functions interface for grid functions
* Adjust test
* Also test with scalar- and matrix-valued dune-functions grid functions as coefficients
* Disable test with `Dune::Functions::ComposedGridFunction` for dune-functions-2.8

After this patch you can no longer pass a `Dune::VirtualFunction` or `VirtualGridFunction`
as diffusivity coefficient, but have to use either the `std::function` or the
`Dune::Functions::GridViewFunction` interface.