adding directional coupler example

[FilipeFcp]

Hi all,

I added a quick example using the directional coupler GDS, as @e-g-melo suggested.

Today, I noticed that there's a very similar notebook in the parameter sweep tutorial, so I focused more on the simulation setup and post-processing in this case.

If you have any suggestions to make it more interesting, feel free to let me know!

[alec-flexcompute]

Great example as always, @FilipeFcp! A couple small points:

1. Also mention that you're creating a field monitor to check out the profile in the middle of the waveguides

2. Above cell 9, add "(if the field decays before the runtime, the simulation will auto shut-off and we will only be billed for the Flex credits used to run)" since this will probably be one of the first examples a new user looks at

3. In the interpolation cell, it might be helpful to add a legend in the plot

That's it, thanks for making these examples very easy to review!

[e-g-melo]

Thanks for this new notebook @FilipeFcp! It is pretty didactic.

Some suggestions:

1. "and create the td.Structures with those material properties" -> "and create the td.Structure with each of those material properties".
2. We don't need the plt.show() after sim.plot_3d() in cell [8].
3. I suggest using dB in the chart of cell [11] to display the -3dB crossing point. The current method also works, though.

Emerson

[tomflexcompute]

Thanks @FilipeFcp . Definitely make sense to have this addition in the example library.

1. The simulation domain size in y can probably be shrunk a bit more to reduce cost?
2. In the plotting, does setting edgecolor="k" actually do anything to the plot?
3. Might be better to plot transmission as a function of wavelength instead of frequency.
4. For printing the transmission, might be better to only show something like 4 significant digits?
5. Probably don't need to add the notebook to docs/features/parameter_sweep.rst?