introduce the ns3::String class, get rid of the string -> Attribute implicit conversion, and get rid of MakeDataRate, port PointToPointNetDevice to Attributes

src/core/string.h

@@ -0,0 +1,32 @@
+#ifndef NS3_STRING_H
+#define NS3_STRING_H
+
+#include <string>
+#include "attribute-helper.h"
+
+namespace ns3 {
+
+class String
+{
+public:
+  String ();
+  String (const char *value);
+  String (std::string value);
+  void Set (std::string value);
+  void Set (const char *value);
+  std::string Get (void) const;
+
+  VALUE_HELPER_HEADER_1 (String);
+private:
+  std::string m_value;
+};
+
+std::ostream & operator << (std::ostream &os, const String &value);
+std::istream &operator >> (std::istream &is, String &value);
+
+VALUE_HELPER_HEADER_2 (String);
+
+} // namespace ns3
+
+
+#endif /* NS3_STRING_H */