Add relative path of map in GroundTruth fixes #278 (#277)

* added map_reference to groundtruth

Author: Ludwig Friedmann

osi_groundtruth.proto

@@ -141,4 +141,13 @@ message GroundTruth
     // - [1] [Proj.4 Projections] (https://proj4.org/usage/projections.html)
     //
     optional string proj_string = 14;
+
+    // Opaque reference of a map.
+    //
+    // \note Origin and orientation of the map have to coincide with the
+    // inertial coordinate frame of the ground truth.
+    //    
+    // \note It is implementation-specific how map_reference is resolved.
+    //
+    optional string map_reference = 15;
 }