LB: Added bell_plotting_helpers.py and s17_plotting_helpers.py, where we can share functionality for ploting. The notebooks are also pushed once, but bell_ploting.ipynb and s17_plotting.ipynb should be kept local (add to .gitignore)

LB: Added bell_plotting_helpers.py and s17_plotting_helpers.py, where we can share functionality for ploting. The notebooks are also pushed once, but bell_ploting.ipynb and s17_plotting.ipynb should be kept local (add to .gitignore)