FunTuple - API changes and simplification of Python helper modules

Hi @amathad, I'm making a simplification here to avoid a packing-unpacking in what is anyway an internal function.

While at it I suggest the renaming tree_name -> tuple_name as I really hope we will move from TTree to RNTuple at some point. I realise this breaks the API and requires updates in DaVinci, but I would rather do it now than 2 months down the line. What do you think?

More details on the API changes in the related DaVinci MR DaVinci!652 (merged).