Add nanoAOD description (for bamboo); modify skimmer
- Rewrite skimmer for synchronization exercise
- Add "nanoAOD description" for bamboo (note: requires https://gitlab.cern.ch/cp3-cms/bamboo/-/merge_requests/92 to be merged, and then update bamboo). This allows to remove unncessary systematic variations (for instance, JEC uncertainty sources only affecting jets in HF doesn't affect the jets we use, it only affects the MET) and be more flexible for the content we add in postprocessing.