[Run2_2017] Transform nested vector branches to flattend+offset vector branches

Merged Alexx Perloff requested to merge github/fork/aperloff/Run2_2017_NestedVectors into Run2_2017

Created by: aperloff

Allow the transformation of vector<vector> branches to be saved as flattened vector branches with an associated vector branch of offsets.

Two unit tests were performed, one keeping the nested vector format and the other flattening the vectors. Flattening the vectors has the effect of decreasing the ntuple size by a little under 1% (0.88% in our 100 event test).

I have not tested the speed change.

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