TTree deprecation and RNtuple

TTree is getting deprecated, especially for writing. RNtuple is the replacement.

We are invited to https://indico.cern.ch/event/1468611/ to present our use cases.

The following features are currently not available in TNtuple:

• Some data types. The list of supported types is shown on slide 7 of the TNtuple slides. Pointers in particular are missing, this will be an issue for Di<>.
• Metadata (UserInfo). This will likely be supported in a better way, discussions are ongoing. I think a realistic push from our side is for a text block in which to put JSON or Info data. Unfortunately this poses a challenge for supporting hadd or merging different metainfos. Maybe the final answer will be TObject with the user expected to implement Merge(), or metadata could be associated with a range of entries.
• Friends, or at least not in the same way as TTree. The logic is getting moved to RDataFrame.

RDataFrame is becoming the preferred ways of accessing ntuples (both TTree and TNtuples). It also supports automatic slicing, and scaling through Dask when called from Python.