Performance improvements, better readme and complete python bindings

include/kpn/variant_node.hpp

@@ -239,11 +239,13 @@ private:
 };
 
 // ── PythonConverter ───────────────────────────────────────────────────────────
+// Specialise for each type you want to use across a PyNetwork.
+// Required members: to_python(const T&), from_python(nb::object).
+// Optional member:  static constexpr const char* type_name = "friendly_name";
+// Built-in specialisations for int/float/double/bool/std::string are provided
+// automatically when you include <kpn/python/auto_bind.hpp>.
 
 template<typename T>
-struct PythonConverter {
-    static_assert(sizeof(T) == 0,
-        "PythonConverter<T> must be specialised for each type used in a PyNetwork");
-};
+struct PythonConverter {};
 
 } // namespace kpn