Add getter for GXFTrajectory track states

Previously, this class had no getter to access the track states stored
within. That's a little bit weird, because it is something we access a
lot. This commit adds that getter.

Tracking/TrkFitter/TrkGlobalChi2Fitter/TrkGlobalChi2Fitter/GXFTrajectory.h

@@ -71,6 +71,7 @@ namespace Trk {
     int numberOfPseudoMeasurements();
     int numberOfOutliers();
 
+    const std::vector<std::unique_ptr<GXFTrackState>> & trackStates() const;
     std::vector<std::unique_ptr<GXFTrackState>> & trackStates();
     std::vector < std::pair < double, double >>&scatteringAngles();
     std::vector < std::pair < double, double >>&scatteringSigmas();

Tracking/TrkFitter/TrkGlobalChi2Fitter/src/GXFTrajectory.cxx

@@ -558,6 +558,10 @@ namespace Trk {
     }
   }
 
+  const std::vector<std::unique_ptr<GXFTrackState>> & GXFTrajectory::trackStates() const {
+    return m_states;
+  }
+
   std::vector<std::unique_ptr<GXFTrackState>> & GXFTrajectory::trackStates() {
     return m_states;
   }