Postprocessing should accept dict JSONs on disk

We should outsource the dicts to disk in prepare_output_file.py to make it easier for users. The paths to the dicts can be passed with argparse. Conerns:

• https://gitlab.cern.ch/HiggsDNA-project/HiggsDNA/-/blob/master/scripts/prepare_output_file.py?ref_type=heads#L133-141
• https://gitlab.cern.ch/HiggsDNA-project/HiggsDNA/-/blob/master/scripts/prepare_output_file.py?ref_type=heads#L146-148
• https://gitlab.cern.ch/HiggsDNA-project/HiggsDNA/-/blob/master/scripts/prepare_output_file.py?ref_type=heads#L153-177