Fixed warnings by adding a proper return statement.

[[Imported from SVN: r889]]

istl/io.hh

@@ -315,6 +315,7 @@ namespace Dune {
       static std::ostream& write(const T& t,  std::ostream& s)
       {
         s<<t;
+        return s;
       }
     };
     template<typename T>
@@ -323,6 +324,7 @@ namespace Dune {
       static std::ostream& write(const std::complex<T>& t,  std::ostream& s)
       {
         s<<t.real()<<" "<<t.imag();
+        return s;
       }
     };
   }