Added handling of None

Solves #55 (closed).

python_binding/graph/pybind_Node.cpp

@@ -169,7 +169,11 @@ void init_Node(py::module& m) {
                 if (pybind11::isinstance<Connector>(arg)) {
                     // Convert Python object to C++ object adn push it ot vector
                     connectors.push_back(arg.cast<Connector>());
-                } else {
+                }
+                else if (arg.is(py::none())) {
+                    connectors.push_back(Connector());
+                }
+                else {
                     throw std::runtime_error("One of the arguments was not a Connector.");
                 }
             }