IterativeSolver: move implementation in Dune::Solvers namespace

Otherwise one gets a confusing error message from the compiler when
dune-istl's Dune::IterativeSolver is imported into the global
namespace before dune-solvers's iterativesolver.hh (and thus also
iterativesolver.cc) is included.

dune/solvers/solvers/iterativesolver.cc

@@ -9,6 +9,9 @@
 
 #include <dune/matrix-vector/genericvectortools.hh>
 
+namespace Dune {
+namespace Solvers {
+
 template <class VectorType, class BitVectorType>
 void IterativeSolver<VectorType, BitVectorType>::check() const
 {
@@ -37,3 +40,6 @@ void IterativeSolver<VectorType, BitVectorType>::writeIterate(const VectorType&
 
     file.close();
 }
+
+} /* namespace Solvers */
+} /* namespace Dune */