Do not require the user to set an error norm

If no norm is given, the error is measured in the l^2 norm

[[Imported from SVN: r12439]]

dune/solvers/solvers/tcgsolver.cc

@@ -11,9 +11,6 @@ void TruncatedCGSolver<MatrixType, VectorType>::check() const
     if (preconditioner_)
         preconditioner_->check();
 
-    if (!errorNorm_)
-        DUNE_THROW(SolverError, "You need to supply a norm to a CG solver!");
-
 }
 
 template <class MatrixType, class VectorType>
@@ -221,10 +218,10 @@ void TruncatedCGSolver<MatrixType, VectorType>::solve()
         // //////////////////////////////////////////////////
         //   Compute error        
         // //////////////////////////////////////////////////
-        field_type oldNorm = errorNorm_->operator()(oldSolution);
+        field_type oldNorm = (errorNorm_) ? errorNorm_->operator()(oldSolution) : oldSolution.two_norm();
         oldSolution -= *x_;
 
-        field_type normOfCorrection = errorNorm_->operator()(oldSolution);
+        field_type normOfCorrection = (errorNorm_) ? errorNorm_->operator()(oldSolution) : oldSolution.two_norm();
 
         if (this->useRelativeError_)
             error = normOfCorrection / oldNorm;