Add color bar limits to permittivity plot

[e-g-melo]

We use the plot_eps function to show the permittivity plot in GUI 2D Chart visualization. Some users have requested custom color bar limits to adjust the color limits as needed.

As we are using WebAssembly to get the permittivity plot, it would be nice to have vmin and vmax parameters in plot_eps. That would also be interesting for Python users. Maybe also include vmin and vmax in plot_structures_eps for uniformity.

Priority: medium