Use relative paths in includes

All files are directly included, instead of a path relative to the dune-uggrid's root director.

Example:
#include "namespace.h"
instead of
#include "low/namespace.h"
in dom/domain.h.

If this is done, we can drop the include_directories command in the CMakeLists.txt and cmake/module/DuneUggridMacros.cmake.