signal flag in jsons

[lucalavezzo]

• added signal boolean in jsons
• lumi is now applied to non-signal MC in the histmaker, alongside the rest of normalizations
• histmaker checks the jsons file as to whether any given sample is a signal, instead of doing awkward string matching
• loader() now checks the metadata to decide if a sample is a signal or not, and applies lumi*xsec if it is
• removed predictSR flag from histmaker, it has been broken for a long time