To the `import_xml` an optional argument `auto_load_definitions` is added. Default is True.

Case:
With commit de8269e4 `import_xml` automatic load(on the fly code generation) the type defintions.

When using your own extension object and enum implementation (with `ua.register_extension_object`  `ua.register_enum`) this is unwanted behaviour.

Previous work flow was:
* import xml
* register own extension object and enum implementations
* for the remain stuff call `load_data_type_definitions()`

To make this possible again an option is required to prevent the auto load definitions when calling the `import_xml` method.