Add native value types

This branch mainly replaces the DataTypeDef and ValueDataType in model.base to use Python classes for all allowed data types and values of those classes instead of the XML/XSD string representation.

The parsing and serialization is handled by the model.datatypes module and invoked by the adapters for serialization and deserialization. Thus, a programmer using PyI40AAS in his application can work with native Python types (like int, float, bool, bytearray, str) and the framework checks type compatibility and handles serialization into the correct string representation.