Provide mutable access to matrix in transfer operator

This is needed to use the new assembleGlobalBasisTransferMatrix(...)
assembler functions which needs access to the internal raw matrix.

dune/solvers/transferoperators/compressedmultigridtransfer.hh

@@ -139,6 +139,11 @@ public:
         return *matrix_;
     }
 
+    /** \brief Direct access to the operator matrix, if you absolutely want it! */
+    TransferOperatorType& getMatrix() {
+        return *matrix_;
+    }
+
     /** \brief Set matrix! */
     void setMatrix(typename std::shared_ptr<TransferOperatorType>& matrix)
     {
@@ -291,6 +296,11 @@ public:
         return *matrix_;
     }
 
+    /** \brief Direct access to the operator matrix, if you absolutely want it! */
+    TransferOperatorType& getMatrix() {
+        return *matrix_;
+    }
+
     /** \brief Set matrix! */
     void setMatrix(typename std::shared_ptr<TransferOperatorType>& matrix)
     {