change ntupliser from class-based content to flat ntuple

[clelange]

There are a few handicaps with the class-based ntuple:

• cannot analyse ntuples without CMSSW/compiling and loading classes
• whenever the classes change, older ntuples cannot be read anymore

I would like to change the ntuple such that it contains only flat branches (and vectors, we probably need vector<vector>). This would mean that everyone needs to change their analysis scripts, but I hope it's not too much work.

Let me know what you think by replying here. We'll probably introduce a branch in parallel to master so that people can have a look before changing the default if people agree.