Adding relative model path to objects fixes #285 (#284)

* Adding relative model_reference to objects

Author: Ludwig Friedmann

osi_object.proto

@@ -21,14 +21,20 @@ message StationaryObject
     //
     optional Identifier id = 1;
 
-    // The base parameters of the vehicle.
+    // The base parameters of the stationary object.
     //
     optional BaseStationary base = 2;
 
     // The classification of the stationary object.
     //
     optional Classification classification = 3;
 
+    // Opaque reference of an associated 3D model of the stationary object.
+    //
+    // \note It is implementation-specific how model_references are resolved to 3d models.
+    //
+    optional string model_reference = 4;
+
     //
     // \brief Classification data for a stationary object.
     //
@@ -308,6 +314,12 @@ message MovingObject
     //
     optional VehicleClassification vehicle_classification = 6;
 
+    // Opaque reference of an associated 3D model of the moving object.
+    //
+    // \note It is implementation-specific how model_references are resolved to 3d models.
+    //
+    optional string model_reference = 7;
+
     // Definition of object types.
     //
     enum Type