Multiple tagger in Rejection per Epoch plot

This MR adds the following:

• Fixing an artifact in the DIPS documentation (still a --tracks flag).
• Renaming the Plotting_settings from the train config files. These settings are only used for plotting the validation metrics plots (plotting_epoch_performance.py). They have a new name now Validation_metrics_settings.
• Inside the new Validation_metrics_settings there are two new options:
  taggers_from_file: Like in the Eval_parameters_validation. Those are the names of the taggers that are available in the .h5 files. All taggers in this list will be plotted to the rejection per epoch plots as horizontal lines.
  trained_taggers: Dict of taggers that are not in the .h5 files but their validation .json file (like for a local trained tagger) is available. Define them here and give the path to the .json file and a label for the legend. The taggers defined here are also going to be plotted into the rejection per epoch plots.
• Rejection per epoch plots are now also available with one rejection per plot. Those are automatically produced when running plotting_epoch_performance.py.
• Unit tests and documentation is provided for all changes.
• A bit of make-the-code-nice-again things.