[Math] Add value printing for LorentzVector

in order to make the interactive experience in python and at the
prompt more rewarding:
```
root [0] ROOT::Math::XYZTVector v
(ROOT::Math::XYZTVector &) (0,0,0,0)
```

math/genvector/inc/Math/GenVector/LorentzVector.h

@@ -753,7 +753,18 @@ namespace ROOT {
 
 } // end namespace ROOT
 
-
+#include <sstream>
+namespace cling
+{
+template<typename CoordSystem>
+std::string printValue(const ROOT::Math::LorentzVector<CoordSystem> *v)
+{
+   std::stringstream s;
+   s << *v;
+   return s.str();
+}
+
+} // end namespace cling
 
 #endif