Resolve "Work on const / non-const in wolf base classes"

Closes #184 (closed)

include/core/trajectory/trajectory_base.h

@@ -53,7 +53,7 @@ class TrajectoryBase : public NodeBase, public std::enable_shared_from_this<Traj
 
     public:
         // factors
-        void getFactorList(FactorBasePtrList & _fac_list);
+        void getFactorList(FactorBasePtrList & _fac_list) const;
 
     protected:
         FrameBaseConstIter computeFrameOrder(FrameBasePtr _frame_ptr);