EnergyNorm: add getMatrix() method

CHANGELOG.md

-# Master (will become release 2.8)
+# Master (will become release 2.9)
+
+- The internal matrix of the`EnergyNorm` can now be accessed by `getMatrix()`.
+
+# Release 2.8
 
 - `UMFPackSolver` can now handle matrices and vectors with scalar entries.
 

dune/solvers/norms/energynorm.hh

@@ -60,6 +60,11 @@ namespace Solvers {
             matrixProvider_ = [=]() -> const MatrixType& { return *matrix; };
         }
 
+        //! \brief get the energy norm matrix
+        const MatrixType& getMatrix() const {
+            return matrixProvider_();
+        }
+
         //! \brief sets to use the current problem matrix of the linear iteration step
         template<class BV>
         void setIterationStep(LinearIterationStep<MatrixType, VectorType, BV>* step) {