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
• ....