Removes Warnings like this:
dune/istl/ilu.hh:49:16: warning: implicitly-declared ‘constexpr Dune::Imp::compressed_base_array_unmanaged<Dune::FieldMatrix<double, 1, 1>, long unsigned int>::RealIterator<Dune::FieldMatrix<double, 1, 1> >& Dune::Imp::compressed_base_array_unmanaged<Dune::FieldMatrix<double, 1, 1>, long unsigned int>::RealIterator<Dune::FieldMatrix<double, 1, 1> >::operator=(const Dune::Imp::compressed_base_array_unmanaged<Dune::FieldMatrix<double, 1, 1>, long unsigned int>::RealIterator<Dune::FieldMatrix<double, 1, 1> >&)’ is deprecated [-Wdeprecated-copy] 49 | for (ij=(*i).begin(); ij.index()<i.index(); ++ij) | ~~^~~~~~~~~~~~~
Some C++ expert should double check this. You cannot trust me here.