[gridptr] Fix compiler warning by explicitly initializing base class

fixes

/home/kilian/Dumux_testing/dune-grid/dune/grid/io/file/dgfparser/gridptr.hh:95:7: warning: base class ‘class std::shared_ptr<Dune::OneDGrid>’ should be explicitly initialized in the copy constructor [-Wextra]
       mygrid_ptr( const mygrid_ptr& other ) { assignObj( other ); }