Only filter and merge in plot_structures_eps with transparency

[tylerflex]

This makes it much faster for most cases, like .plot() with many structures

[tylerflex]

plotting Simulation with sim.medium.permittivity=2 and structures with medium permittivity ranging from 1-3.

plot()

does not filter and merge

plot_eps() with no alpha:

does not filter and merge

plot_eps() with alpha:

filters and merges

[momchil-flex]

What's your thinking about plot_field eps overlays? Since monitors will usually only cover a subspace of the simulation that hopefully won't have way too many structure intersections, it's tempting to leave it on by default. And it already accepts an eps_alpha argument so it can be turned off if eps_alpha=0? I think the only thing to make sure then is that this indeed turns the overlay off, e.g. in simulation.plot_structures_eps, if alpha == 0: return ax

[tylerflex]

I dont think we can assume monitors cover subspaces that dont have too many structure intersections unfortunately.

But I dont think we have any other option but to still do the filtering and merging for overlays.

And yea if eps_alpha=0 we just dont call simulation.plot_structures_eps() (or we can do it if alpha=0 in that function call too).

[momchil-flex]

I guess my question is simply should we then disable the overlay in field plotting by default?