[ENH] - Plot updates & extensions

[TomDonoghue]

Some additions & tweaks to plotting including:

• add a plot for multiple time series (plotted with a vertical offset)
• add a combined 'timeseries & psd' plot
• style tweaks that allow for settings ticks and ticklabels through plot_style
• update styling descriptions and implementation, following fooof updates ([MNT] - Make grid an updateable argument in PSD plots fooof-tools/fooof#300)
• add tight_layout as a custom plot style, meaning it can be passed in as an argument to turn on/off, and also apply with warning suppression, to stop annoying warnings that the plot layout has changed

The upshot of the style update is that something like this now works:

# Plot with a neurodsp plot function with `plot_style`, turning off xticks
plot_function(data, xticks=[])

Note: the tests on newer versions of python were failing on a doctest, where an answer that used to come out at 2.0 is now 2.000000000000026 (or something) - so I added rounding to this check.

[ryanhammonds]

Looks good! I added a few optional comments. Something to think about for the future though is a separate plotting package the signals/psds, as the plotting functionality here grows.

[TomDonoghue]

Thanks for checking @ryanhammonds! I made some updates based on your comments, if you want to check the update for the multi plot in particular (since what the API for that should be is not so obvious).

And yeh, I agree - as things grow, we should definitely consider "what belongs where" and if/when we might consider splitting things out from ndsp. I've been meaning to do a proper sweep of code for a bit, so maybe let's check in on this sometime soon, as we check in on what goes into 2.3, what goes after, and any things we need to figure out on the roadmap otherwise.