Draft: Dev

• move unfolding from Core to Darwin (!127)
• revisit (or even remove?) UserInfo (could be replaced with nlohmann::json, which is available through ROOT)
• allow multiple git repos in MetaInfo (#99)
• have a global Proto namespace (\to Proto::Darwin, Proto::Beagle)
• upgrade to latest ROOT to include fixes related to TTree friends (!129)
• improve treatment of variations (!136 (merged))

This MR implies significant changes downstream, especially in https://gitlab.cern.ch/cms-analysis/general/DasAnalysisSystem/Core.