add signif_kwargs and signif_alpha to psd.plot()

[jordanplanders]

When I construct a psd plot that includes multiple timeseries, it becomes confusing to track which significance line corresponds to which timeseries unless they are color coded to their parent timeseries. However, if they are color coded, having two lines of the same color can be a bit visually confusing (the line style helps, but still).

I would like the ability to specify the alpha value of the significance line (signif_alpha), in the same way you can specify signif_clr. Similarly, it would be logical to update psd.plot to accept signif_kwargs just like plot_kwargs and beta_kwargs.

[CommonClimate]

I'd need to see an example, but in principle I'm supportive. Let's triage this on Friday. It's much lower on the priority list than having a geologic timescale on plots.