Add xAOD reduction script

This adds a script to make a reduced AOD. I mostly wanted this to add unit tests, e.g. #60 (closed).

It might also be useful so we can make the PHYSVAL-based test files directly from production datasets. The script also slims down the inputs quite a bit by dropping branches: a quick local test reduces the PHYSVAL input for the pflow pipeline from 6 MB down to 1.3 MB.

I also took the opportunity to clean up the testing scripts a bit: there were some outdated things like the GNN test and the ability to run EventLoop.