Switch function interface in `PortablePixelMap`

* Let the function implement the dune-functions interface
* Require dune-functions in `exportPixelMap()` member function
* Adjust `ppmtest.cc`

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.