WIP: Viz for spatial filters

[Genuster]

What does this implement/fix?

Adds SpatialFilter container for visualisation of filters, patterns and eigenvalues of GED-based algorithms (CSP, SPoC, SSD, XdawnTransformer) and other linear models (LinearModel).

• implement plot_filters, plot_patterns, plot_scree
• make possible to plot inverse_transform'ed filters/patterns in case of pipeline (as get_coef does)
  • make get_coef work with arbitrary step in the pipeline (not only the last LinearModel)
• cover tests
• replace CSP's plots with internal functions

[wmvanvliet]

I'm all for adding nice visualizations for filters/patterns. Suggestion: would a MixedIn pattern make sense here instead of the class you have now?

[Genuster]

Cheers @wmvanvliet!

My original thought was to make it usable with external spatial filters (PyRiemann transformers, for example).
I also realize now if the relevant transformer is in the middle of the pipeline and I want visualize its filters/patterns after inverse-transform, it would make sense to use a plain function that would instantiate the Viz class instead of adding it as a method to GED/LinearModel.
If I go with Mixin this case would be problematic, as far as I can tell.

Does that make sense? If you have an idea how we can work around it with Mixin, I'd be glad to hear