Small Code Improvements

[PythonFZ]

Test with

• better error messages when using Node.write_graph
• add Node.convert_notebook and Node.write_graph(no_notebook=True) for splitting up conversion of Jupyter Notebooks and writing the Graph. Might be replaced by Import directly from Jupyter Notebooks #216 if possible.
• add dvc.checkpoints() for checkpoint support (see https://dvc.org/doc/user-guide/experiment-management/checkpoints)
• internal restructure using dataclasses for ZnType and zntrack.json / params.yaml in one place instead of having strings everywhere

[SamTov]

Some comments on code structure / formatting but otherwise it seems good.