FunTuple - API changes

Depends on Analysis!862 (merged).

API changes are done at this point to be more ready for a replacement TTree -> RNTuple at some point in the near future (through tests and some "R&D" will be required when RNTuple becomes available in Gaudi).

Main changes are:

• Renaming tree -> tuple (to be more generic and less attached to TTree per se).
• Renaming branch -> field (fields are indeed used in RTuple).

Note that many places in the documentation and code comments often say "fields (branches)" to ease the transition for users.