Implementation of semileptonic selection and triggers

Created by: mmarchegiani

The semileptonic selection is implemented. A dedicated library is defined to apply the triggers, where the trigger application is parametrized by the final state. The semileptonic triggers are updated accordingly for the 2018UL dataset. A future PR will include also the triggers for the 2016UL and 2017UL datasets. The class Dataset is updated to handle the output of the dasgoclient command and save the file list correctly.

Additionally, a new JSON scheme is implemented to build the dataset and bookkeep the samples' names and eras.

Finally, Configurator is modified such that the script scripts/plot/make_plots.py will plot the histograms reading the most recent version of the output, unless specified otherwise.