add postopus introduction in benzene tutorial
Description
This MR adds a section on postopus in the visualisation tutorial, with a python snippet to generate a plot of density at z=0 plane. The code snippet was initially suggested by @sohlmann
This MR adds a section on postopus in the visualisation tutorial, with a python snippet to generate a plot of density at z=0 plane. The code snippet was initially suggested by @sohlmann