New box-and-whisker renderer and updated violin renderer

[alex-hhh]

Add a new box-and-whisker renderer and update the violin renderer to use a #:width parameter instead of the #:skip and #:gap arguments. Also violin renderer now uses a single set of line parameters (color, width and style) instead of two.

Updated tests and documentation to provide a more comprehensive example.

[racket-discourse-github-bot]

This pull request has been mentioned on Racket Discussions. There might be relevant details there:

https://racket.discourse.group/t/box-and-whisker-plot-and-updated-violin-plot/622/1