Document interactive plotting use

[danielhollas]

On Fedora 39 I am getting the following warning when running the script with uv (uv run promdens.py)

../promdens.py:813: UserWarning: Matplotlib is currently using agg, which is a non-GUI backend, so cannot show the figure.
  plt.show()

We should document what is needed to make this work.

Also, I think it might be perhaps worthwhile to implement a new option that turns on/off the interactive plotting, which will be useful for tests.

[danielhollas]

With a system python and dependencies installed manually into venv I see a slightly different message

/home/hollas/projects/wigner_pulse_jirka/ics_with_laser_pulse/./promdens.py:814: UserWarning: FigureCanvasAgg is non-interactive, and thus cannot be shown

Some hints are here, probably need to install additional GUI backend separately

https://stackoverflow.com/questions/56656777/userwarning-matplotlib-is-currently-using-agg-which-is-a-non-gui-backend-so

Official matplotlib docs
https://matplotlib.org/stable/users/explain/figure/backends.html