* Switch to dune-functions interface for grid functions
* Modernize constructor: You now have to pass a `QuadratureRuleKey`
  describing how to integrade the passed function instead of
  a raw quadrature rule order for the whole integrant.
* Reduce magic:
  * This no longer works like a simple `H1FunctionalAssembler`
    if you pass Subgrid `GridFunction`s. In this use case
    use a plain `H1FunctionalAssembler` instead.

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