Adding plotting upgrades

This MR adds the following:

• New options for the plotting_umami.py plots:
  • dpi: This sets the DPI value of the plots. Can be given in the config file in the plotting_settings. Documentation is added.
  • labelFontSize for the pT_vs plots: Setting the fontsize for the axes labels and ticks.
  • legFontSize for the pT_vs plots: Setting the fontsize for the legend.
• Adding the option tagger_label. This is added in the train config in the Validation_metrics_settings. This is the label for the fresh trained model for the legend of the rejection vs epoch plots. If it is not given or None, the tagger from NN_structure is used (for backward compatibility). Documentation is added. Unit tests are adapted.
• Removing double loop function over files from the input_vars_tools. This feature is deprecated because it is already supported now by the LoadTrksFromFile and LoadJetsFromFile functions which are used here for loading.
• Adding variable fontsize to makeATLAStag.