Add component selection to plot_eps

[e-g-melo]

Current

When we call plot_eps and the simulation has anisotropic material models, the eps limits are calculated using eps_model which in turn, takes the average between the eps components on the main diagonal. So, it is not possible to visualize a specific permittivity component.

For example, we have this anisotropic material model with a main diagonal of [15, 10, 5].

When we plot it, the maximum permittivity is 10.

Requirement

Considering we have introduced some anisotropic materials in our material library and we have some users working with LiNbO3, it would be interesting to allow the users to specify which eps component they want to plot. We could keep the average of the mais eps diagonal as the default for backward compatibility. Maybe something like plot_eps(component="xx") and the user can select between xx, yy, zz, or None. If the medium is fully anisotropic, maybe we could allow for xx, xy, xz, yx, yy, yz, zx, zy, zz, or None.

[tylerflex]

so I think this would be a good "first" issue for new employees. One would need to trace through the plotting, adjust the structure eps plotting to make use of this eps_diagonal, and then make everything work

https://github.com/flexcompute/tidy3d/blob/develop/tidy3d/components/medium.py#L4982-L4994

Unless you think it's urgent enough that we need someone to implement right away?

[e-g-melo]

We don't have complaints about it, so it is not urgent at the moment.