Group ntuples branches by systematics
To try to improve the reading of ntuples created by the central CP algorithms, it is useful to group output branches by systematics. As usually one always reads specific systematic branches together this should reduce the I/O operations needed to read the trees.
I made relatively minor changes to the algorithm:
- Make a common helper function to register a branch.
- First loop through nominal systematics and ensure they are created, otherwise fail the job.
- Then loop over the rest of systematics.
P.S. As indentation changed this MR looks larger than it really is.