TupleOp, proxies, and backend refactoring from https://gitlab.cern.ch/cp3-cms/bamboo/-/merge_requests/148
Rebased and reordered/split, to facilitate testing of these changes separately (they should be transparent to the user code).
No changes are expected with these changes, but some were reported by @swertz , so this will stay open until those are understood.
Edited by Pieter David