Make getStructure(), getFrameStructure and getStateStructure more uniform

include/core/problem/problem.h

@@ -83,7 +83,7 @@ class Problem : public std::enable_shared_from_this<Problem>
         // Properties -----------------------------------------
     public:
         SizeEigen getDim() const;
-        StateStructure getFrameStructure() const;
+        const StateStructure& getFrameStructure() const;
 
     protected:
         void appendToStructure(const StateStructure& _structure);

include/core/processor/is_motion.h

@@ -29,8 +29,8 @@ class IsMotion
         virtual VectorComposite getState(const StateStructure& _structure = "") const = 0;
         virtual VectorComposite getState(const TimeStamp& _ts, const StateStructure& _structure = "") const = 0;
 
-        std::string getStateStructure(){return state_structure_;};
-        void setStateStructure(std::string _state_structure){state_structure_ = _state_structure;};
+        const StateStructure& getStateStructure ( ) { return state_structure_; };
+        void setStateStructure(std::string _state_structure) { state_structure_ = _state_structure; };
         void addToProblem(ProblemPtr _prb_ptr, IsMotionPtr _motion_ptr);
    
     protected:

src/problem/problem.cpp

@@ -543,7 +543,8 @@ SizeEigen Problem::getDim() const
 {
     return dim_;
 }
-StateStructure Problem::getFrameStructure() const
+
+const StateStructure& Problem::getFrameStructure() const
 {
     return frame_structure_;
 }