Updates required for QAT

This MR includes:

• Abs operator;
• Hook system for Node (as suggested in #98 @thibaultallenet).
• Update of SinglePassGraphMatching to match no connection (with the special node keyword $).
• Solved some SinglePassGraphMatching issues and added more complex unit tests.
• Added missing inChannels() method for FC operator;
• Added matchFrom() method to SinglePassGraphMatching in order to match a query only for a specific starting node;
• Fix issue #87 (closed).

include/aidge/data/Tensor.hpp

@@ -312,6 +312,18 @@ class Tensor : public Data,
      */
     Tensor sqrt() const;
 
+    /**
+     * @brief Element-wise abs operation for Tensor.
+     * @return Tensor
+     */
+    Tensor abs() const;
+
+    /**
+     * @brief Mean operation for Tensor.
+     * @return Tensor
+     */
+    Tensor mean() const;
+
     ~Tensor() noexcept;
 
 public: