add visualizer to make boxplots

[nbokulich]

Improvement Description
maturity-index once generated boxplots, which were replaced by a volatility plot sometime around the 2018.8 release. A forum user has requested this visualization — it could be nice to have this as a generic plotting utility.

Current Behavior
No boxplots.

Proposed Behavior
Make boxplot plotting utility that accepts any metadata. Similar to volatility, but boxplots instead. Alternatively, boxplots could be an optional overlay in the volatility plots.

References
forum xref