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.cc

@@ -0,0 +1,45 @@
+#include "string.h"
+
+namespace ns3 {
+
+String::String ()
+  : m_value ()
+{}
+String::String (const char *value)
+  : m_value (value)
+{}
+String::String (std::string value)
+  : m_value (value)
+{}
+void 
+String::Set (std::string value)
+{
+  m_value = value;
+}
+void 
+String::Set (const char *value)
+{
+  m_value = value;
+}
+std::string 
+String::Get (void) const
+{
+  return m_value;
+}
+
+std::ostream & operator << (std::ostream &os, const String &value)
+{
+  os << value.Get ();
+  return os;
+}
+std::istream &operator >> (std::istream &is, String &value)
+{
+  std::string str;
+  is >> str;
+  value = String (str);
+  return is;
+}
+
+VALUE_HELPER_CPP (String);
+
+} // namespace ns3