Write file attribute to XML file.

The write() method now writes the file attribute to the XML file.
Implement size() method inline.
Add haspar() method.

The haspar() method checks whether a parameter of a given name does exist in the model.
Add at() and hasmodel() methods.

The at() methods provide model access by index with index validity checking.

The hasmodel() method checks whether a model with a given name exists in the container.
Add registry and GModels unit tests.
Adapt to new model interface.
Adapt to new model interface.
Update after review of model classes.