plotting Waveguide mode components

[lukasc-ubc]

It would be nice to be able to plot only Ex, Ey, or Ez, rather than all three as displayed by the mode.show() method.

I looked at the plot_mode function, and I see that plotting all three is hard coded. Adding an optional parameter to the function would be an easy addition.

https://github.com/HelgeGehring/femwell/blob/main/femwell/maxwell/waveguide.py