Decorate postprocessed NanoAODs

Fixes #35 (closed)

Still to do:

• testing
• also vary bTagSF if available (see #37 (closed) )
• pick up some optimisations
• find a way to selectively disable variations can pick up the originals, but the decorations may need to take more arguments (see #37 (closed) for more discussion)
• puWeight_up/down from branch