Perform house keeping before new release

[ahelm]

We have a lot of code associated with the previous release that is not used or was moved to other objects. Before a new release, we should focus on housekeeping:

• [ ] cleanup nata.types
• [ ] cleanup unused objects and files in nata.containers
• [ ] cleanup unused objects in nata.plugins
• [ ] cleanup tests associated with the previous API
• [ ] cleanup warning messages
• [ ] check jupyter notebooks for completeness and that all run correctly
• [ ] use 00_overview.ipynb, 01_some_notebook.ipynb, ... for the notebook naming
• [ ] turn example notebook for particles into a proper notebook with examples
• [ ] ....