Draft: Feature/merge

See #15

Wishlist:

• concatenate
• hadd (compare metainfo of input files and adapt git flags accordingly)
  • compare flags one by one (\to all must match to be reproducible)
  • combine historyies and add one more line for merge
  • compare preseeds: should be identical for a given data set split into many files, but different for two data sets (unsure how to combine them in a reproducible way at this stage)
  • compare software versions (\to all must match to be reproducible)
  • compare corrections (idem)
  • compare variations (\to crash if not identical)
  • compare/combine datasets (this block has been newly introduced in DAS, but should be introduced in ProtoDarwin too)
• rivet-merge
• JSON? INFO?
• Boost.Histograms?