Integrate openpmd-scipp into the main API

[franzpoeschel]

This merges the code from https://github.com/pordyna/openpmd_scipp into the main API such that it can be used by series.to_scipp().

TODO:

• Documentation (fields)
• Documentation (particles) COMMENT: The TODO there refers to particles not being even implemented yet, I think?
• Clean up the example
• Have a look at the rest of what is in the repository, see what needs to / can be transferred
• CI integration
• Licensing headers
• remove Jupyter Notebook
• check image sizes