move columnar accessors from tool to a separate struct

Nils Erik Krumnackrequested to merge
krumnack/athena:muon_sf_acc into main

I ran into this with a couple of other columnar tools: Essentially the root dictionaries are not happy with the columnar accessors as part of the class itself. The most practical/immediate solution is to move all of them into a separate struct that is not visible to the dictionary.

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