Unify sample definitions

[lucalavezzo]

Follow up issue from PR #664.

It would be nice to unify xsecs, BRs, filepaths, lumis, etc. for each sample in one dictionary:

{
 "<era>": {
      "<sample>": {
        "xsec": ....
        "filepaths": [...],
        "lumicsv": "filename"
      }
    }
}

This information is now scatted across a couple of different scripts.