AMcylindrical geometry: showing the Field diagnostic on both half planes

[Z10Frank]

A major advantage of the Field diagnostic compared to Probes is that the x axis can be changed with the propagation of the moving window if the user selects moving=True, but it only shows half plane in "AMcylindrical" geometry.

On the other hand, Probes allow to visualise both half planes (e.g. the whole xy plane) if properly positioned, but do not allow the moving=True because this would not be manageable with all the possible Probes positioning.

One compromise to couple the advantages of both diagnostics would be to add an argument in the Field diagnostic, e.g. show_also_theta_plus_pi, to show also the plane at the selected theta+pi when set to True, and visualising both half-planes combined.