Added accessors for navigation state index.

VecGeom/navigation/NavStateIndex.h

@@ -100,6 +100,14 @@ public:
     return sizeof(NavStateIndex);
   }
 
+  VECCORE_ATT_HOST_DEVICE
+  VECGEOM_FORCE_INLINE
+  NavIndex_t GetNavIndex() const { return fNavInd; }
+
+  VECCORE_ATT_HOST_DEVICE
+  VECGEOM_FORCE_INLINE
+  void SetNavIndex(NavIndex_t ind) { fNavInd = ind; }
+
   VECCORE_ATT_HOST_DEVICE
   int GetObjectSize() const { return (int)sizeof(NavStateIndex); }