Add python bindings for the RNTuple backend

[tmadlener]

BEGINRELEASENOTES

• Add python bindings for the RNTuple reader and writer
• Make podio-dump understand RNTuple based files
• Fix missing storage of datamodel definitions for RNTuple based files

ENDRELEASENOTES

Tried to get this to work with spack locally, but currently stumbling over root-project/root#11797 so I cannot build podio inside spack with ENABLE_RNTUPLE. Curious to see in how many of our CI environments this works.

[tmadlener]

I see very similar errors locally:

    File "/home/runner/work/podio/podio/tools/podio-dump", line 172, in <module>
      main(clargs)
    File "/home/runner/work/podio/podio/tools/podio-dump", line 111, in main
      reader = get_reader(args.inputfile)
    File "/home/runner/work/podio/podio/python/podio/reading.py", line 74, in get_reader
      return root_io.RNTupleReader(filename)
    File "/home/runner/work/podio/podio/python/podio/root_io.py", line 42, in __init__
      self._reader = podio.ROOTNTupleReader()
  TypeError: cannot instantiate incomplete class 'podio::ROOTNTupleReader'

I am not sure why this doesn't work. I can instantiate a ROOTNTupleReader outside of the test environment, but not inside it. My suspicion is (once again) a weird interplay between installed dictionaries and newly built ones for the tests. I haven't been able to resolve this locally due to the spack issues mentioned at the top.

[tmadlener]

This is now working for me in a local installation, and I think it should also work for a key4hep based stack. I am not entirely sure how things look like in other environments.

[jmcarcell]

Maybe not needed now because there aren't that many choices but all of this can be changed by a basically one liner instead of all the if - return:

return {RootFileFormat.TTREE: root_io.LegacyReader, RootFileFormat.RNTUPLE: root_io.RNTupleReader, 
        RootFileFormat.LEGACY: root_io.LegacyReader}[_determine_root_format(filename)](filename)

[tmadlener]

I would keep it as it is right now, because it is easier to read, and we could think about replacing it with match /case once we bump our minimum python version to 3.10.

[jmcarcell]

I tested it with an rntuple with EDM4hep and it worked fine

[tmadlener]

Thanks for testing.