Const correctness in iterative solvers

The constructors of IterativeSolvers took std::shared_ptr<LinearOperator<X,Y>> and std::shared_ptr<ScalarProduct<X>> as arguments, where the const version actually should suffice, since the corresponding methods are marked as const.

This MR changes the arguments to std::shared_ptr<const LinearOperator<X,Y>> and std::shared_ptr<const ScalarProduct<X>> and adapts the SolverFactory such that solvers can be created from a const operator.

This is a change of the interface but should not lead to problems due to the implicit conversion of std::shared_ptr<T> to std::shared_ptr<const T>.

dune/istl/test/solvertest.cc

@@ -68,7 +68,7 @@ int main(int argc, char** argv)
   typedef Dune::MatrixAdapter<BCRSMat,BVector,BVector> Operator;
 
   BCRSMat mat;
-  Operator fop(mat);
+  const Operator fop(mat);
   BVector b(N*N), x(N*N);
 
   setupLaplacian(mat,N);