[forms][cleanup][doc] Cleanup code structure of Dune::Fufem::Forms and improve documentation

• Remove interdependence of most headers.
• User functionality is now almost completely contained in the new userfunctions.hh header that provide a convenience interface to the underlying low level classes.
• Hide special cases of the user interface in Dune::Fufem::Forms::Impl and only expose a single user entry point for each feature.
• Group components of Dune::Fufem::Forms in doxygen.
• Add a rudimentary main page for the doxygen documentation.
• Add at least a rudimentary documentation for each low lever implementation class.
• Most importantly: The user interface of Dune::Fufem::Forms is now fully covered by the doxygen documentation.