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Describe the bug

I tried to create "additional" datasets for one RooFit sample. They do not show up in the XML file (and it seems also not in the root file of the model). When discussing this with @cburgard , we found that the fAdditionalData member of Channel is supposed to hold additional datasets to be added to the XML, but is never used in `Channel::XML.

Expected behavior

The additional datasets also get exported to XML/rootfile/etc.

To Reproduce

Add an additional data sample to a workspace, save it.

I have modified one of the python examples so that it should reproduce this issue.
It was first discovered in our analysis framework.
histfactoryxmlexport.py.txt

Setup

Both with ROOT 6.20/06 from AnalysisBase,21.2.204 and ROOT 6.24/06 from LCG_101_ATLAS_20

Additional context