Histogramming using a dictionary for automatic creation and writing

Introducing a dictionary for the histograms which is configurable through a json and then histograms are automatically created and added to the histograms without the need to write the full list for every .Write() command. However, we only save on the Write (and Clear, which we don't use at the moment).

It is working now, ready to merge.

Any new histograms can easily be added just in the cfg/histograms.json file. There is one convention: "name" of the histograms using MCParticles collection should start with "true" and then they are not initialized/filled/written for the case of the background prodids.