Add assertion: Vectors must have identical size

dune/solvers/norms/twonorm.hh

@@ -35,6 +35,7 @@ class TwoNorm : public Norm<VectorType>
         //! Compute the norm of the difference of two vectors
         virtual double diff(const VectorType& f1, const VectorType& f2) const
         {
+            assert(f1.size() == f2.size());
             double r = 0.0;
 
             for (size_t i=0; i<f1.size(); ++i)