Code to abstract how datatypes are returned to enable multiple return types.
This code enables pandas df, awkward array, ROOT file/TTree.
Most of the movement is in the back end for the xAOD processor and won't affect others.
Some code changes, however, happen in the front end. In particular, how the user requests a particular datatype has now been normalized: an AST is generated and it is up to the back end to figure out how to generate that request.
Code to abstract how datatypes are returned to enable multiple return types.