Make track parameters getter const

There is really no reason for this getter method to not be const. The
returned pointer is to a const value, and the non-const qualification
makes it impossible to use this extremely useful getter in non-const
contexts.

Tracking/TrkFitter/TrkGlobalChi2Fitter/TrkGlobalChi2Fitter/GXFTrackState.h

@@ -47,7 +47,7 @@ namespace Trk {
     const MeasurementBase *measurement(void);
 
     void setTrackParameters(std::unique_ptr<const TrackParameters>);
-    const TrackParameters *trackParameters(void);
+    const TrackParameters *trackParameters(void) const;
     
     GXFMaterialEffects *materialEffects();
     const Surface *surface() const;
@@ -150,7 +150,7 @@ namespace Trk {
     return m_covariancematrix;
   }
 
-  inline const TrackParameters *GXFTrackState::trackParameters(void) {
+  inline const TrackParameters *GXFTrackState::trackParameters(void) const {
     return m_trackpar.get();
   }