Convert ScatterVisualizer to a mixin

[rebeccabilbro]

Describe the solution you'd like
Convert ScatterVisualizer, currently in contrib, into a YB mixin.

Is your feature request related to a problem? Please describe.
Besides ScatterVisualizer many YB visualizers implement some kind of scatterplot:

• RadViz
• Dispersion
• Manifold
• JointPlotVisualizer
• PCADecomposition
• ResidualsPlot
• PredictionError
• TSNE

And it's likely that future visualizers may also require some form of scatterplot. In order to implement these in a more uniform and predictable way, it would be convenient to convert the current ScatterVisualizer, which was recently moved to contrib, into a YB mixin that could both serve as a base for the above visualizers and support future development.

See discussion in #475

[bbengfort]

See also #676 and #677 which both would influence this mixin.

[bbengfort]

With the DataVisualizer base class I think we've moved past the need for this mixin; or it would have to take a very different form with our detect target type utilities. Closing in favor of a new approach to this.