Piotr's Tutorial 7 on hyperbolic problems

Dune course is approaching, time to clean things up. This MR allows us to better review and comment on the code.

tutorial07/src/CMakeLists.txt

-foreach(dim 2 3)
-  add_executable("tutorial07-${dim}d" tutorial07.cc)
-  target_compile_definitions("tutorial07-${dim}d" PUBLIC "GRIDDIM=${dim}")
-endforeach()
+add_executable("tutorial07-maxwell" tutorial07-maxwell.cc)
+add_executable("tutorial07-acoustics" tutorial07-acoustics.cc)
 
-dune_symlink_to_source_files(FILES tutorial07.ini)
+dune_symlink_to_source_files(FILES tutorial07-maxwell.ini)
+dune_symlink_to_source_files(FILES tutorial07-acoustics.ini)