Add plot_violin

[aloctavodia]

No description provided.

[imperorrp]

The violin plot in its current implementation in #85 overlays the densities of multiple models when multiple models are provided on both 'left' and 'right' sides (although these sides can be toggled on/off with plot_kwargs). Do we want to keep this behavior or @OriolAbril's suggestion to have the first two models be mapped to the 'left' and 'right' sides of the violin plot, and then perhaps overlay the additional models beyond 2, or something else entirely?

[OriolAbril]

My proposal is to raise an error if there are more than two models. I think overlaying things on a violin plot is a terrible idea and should never happen.

[imperorrp]

Okay, I'll add in a check and make the modifications to do so