[bugfix] Use const references in TruncatedBlockGSStep constructor

There's no need to pass the matrix and rhs non-const.

dune/solvers/iterationsteps/truncatedblockgsstep.hh

@@ -34,7 +34,7 @@ public:
     {}
 
     //! Constructor with a linear problem
-    TruncatedBlockGSStep(MatrixType& mat, VectorType& x, VectorType& rhs, int innerLoops=1)
+    TruncatedBlockGSStep(const MatrixType& mat, VectorType& x, const VectorType& rhs, int innerLoops=1)
         : LinearIterationStep<MatrixType,VectorType>(mat, x, rhs),
         innerLoops_(innerLoops)
     {}