[densematrix] fix densematrix column iterator type for row_reference != row_type&

Note remove_reference is necessary, because you cannot extract the iterator type from
row_reference directly in case: row_reference == row_type&

dune/common/densematrix.hh

@@ -251,7 +251,7 @@ namespace Dune
     //! rename the iterators for easier access
     typedef Iterator RowIterator;
     //! rename the iterators for easier access
-    typedef typename row_type::Iterator ColIterator;
+    typedef typename remove_reference<row_reference>::type::Iterator ColIterator;
 
     //! begin iterator
     Iterator begin ()
@@ -286,7 +286,7 @@ namespace Dune
     //! rename the iterators for easier access
     typedef ConstIterator ConstRowIterator;
     //! rename the iterators for easier access
-    typedef typename row_type::ConstIterator ConstColIterator;
+    typedef typename remove_reference<const_row_reference>::type::ConstIterator ConstColIterator;
 
     //! begin iterator
     ConstIterator begin () const