enable using AdditionalInfo as Gaudi::Property

Enable using AdditionalInfo as Gaudi::Property. For example, to be added as member to a Transformer and as such enable passing an extra value to it.

Example usage case in Rec!2773 (merged).

Event/RecEvent/include/Event/ProtoParticle.h

@@ -186,6 +186,13 @@ namespace LHCb {
           10000 // The last official flag value. Private user variables should use flags greater than this value
     };
 
+    // functions necessary for AdditionalInfo to work with Gaudi::Property
+    friend std::string   toString( additionalInfo e );
+    friend std::ostream& toStream( additionalInfo e, std::ostream& os ) {
+      return os << std::quoted( toString( e ), '\'' );
+    }
+    friend StatusCode parse( additionalInfo& bt, std::string in );
+
     /// Copy constructor. Creates a new ProtoParticle with the same physics information
     ProtoParticle( const LHCb::ProtoParticle& proto )
         : KeyedObject<int>()