Legend support

[OriolAbril]

At the time of writing, we have some legend support for bokeh, relatively good legend support for matplotlib and no legend support for plotly.

We should explore better APIs and extend legend support. At a minimum, pc.add_legend("dim_name") should work on all backends and generate acceptable legends.

Once add_legend is working better, it should be called automatically for several plots, including (but might not be limited to):

• plot_psense_dist -> for alpha
• plot_energy
• plot_xyz when model is mapped to color