added some standard type exports

[[Imported from SVN: r5689]]

dune/solvers/operators/nulloperator.hh

@@ -32,6 +32,10 @@ class NullOperator
     public:
         //! export the block type
         typedef BlockType block_type;
+        //! export the field type
+        typedef typename block_type::field_type field_type;
+        //! export the size type
+        typedef size_t size_type;
 
         //! Default constructor
         NullOperator(){}