Update 'Operator::setOutput()' and overrides to be const

Context

Operator::setOutput should precicely be const as it is intended to change the object mOutput smart pointers are pointing to but not the pointer

include/aidge/operator/Operator.hpp

@@ -109,7 +109,7 @@ public:
      * The pointer itself is not changed, thus keeping the current connections.
      * @param inputIdx Index of the input to set.
      */
-    virtual void setOutput(const IOIndex_t outputIdx, const std::shared_ptr<Data>& data) = 0;
+    virtual void setOutput(const IOIndex_t outputIdx, const std::shared_ptr<Data>& data) const = 0;
     virtual std::shared_ptr<Data> getRawOutput(const IOIndex_t outputIdx) const = 0;
 
     std::shared_ptr<Hook> getHook(const std::string& hookName) {