add getter for rgb

gtsam/slam/dataset.h

@@ -226,6 +226,9 @@ struct SfmTrack {
   std::vector<SfmMeasurement> measurements; ///< The 2D image projections (id,(u,v))
   std::vector<SiftIndex> siftIndices;
   
+  /// Get RGB values describing 3d point
+  Point3 rgb() const { return Point3(r, g, b); }
+
   /// Total number of measurements in this track
   size_t number_measurements() const {
     return measurements.size();