Make getNodes() return a const ref

include/aidge/graph/GraphView.hpp

@@ -208,7 +208,7 @@ public:
      * @brief Get the Nodes pointed to by the GraphView object.
      * @return std::set<NodePtr>
      */
-    inline std::set<NodePtr> getNodes() const { return mNodes; }
+    inline const std::set<NodePtr>& getNodes() const { return mNodes; }
 
     /**
      * @brief Get the operator with the corresponding name if it is in the

src/graph/Connector.cpp

@@ -39,7 +39,7 @@ std::shared_ptr<Aidge::GraphView> Aidge::generateGraph(std::vector<Connector> ct
             graph->add(nodesToAdd.back());  // only add, connection already done
                                             // between nodes
             std::vector<std::shared_ptr<Node>> parents = nodesToAdd.back()->getParents();
-            std::set<std::shared_ptr<Node>> alreadyAdded = graph->getNodes();
+            const std::set<std::shared_ptr<Node>>& alreadyAdded = graph->getNodes();
             for (std::shared_ptr<Node> parent : parents) {
                 if (alreadyAdded.find(parent) == alreadyAdded.end()) {
                     buffer.push_back(parent);