Make toString pure virtual and remove data ctor binding.

include/aidge/data/Data.hpp

@@ -52,7 +52,7 @@ public:
         return mType;
     }
     virtual ~Data() = default;
-    virtual std::string toString() const;
+    virtual std::string toString() const = 0;
 
 private:
     const std::string mType;

python_binding/data/pybind_Data.cpp

@@ -29,8 +29,7 @@ void init_Data(py::module& m){
     .value("UInt64", DataType::UInt64)
     ;
 
-    py::class_<Data, std::shared_ptr<Data>>(m,"Data")
-    .def(py::init<const std::string&>());
+    py::class_<Data, std::shared_ptr<Data>>(m,"Data");
 
 
 }