Allow `width` to be set per box in `boxplot`

[EdsterG]

Description

Allows width to be set per box. Not sure if this is a bug fix or new feature, classified it as a bug fix since colors can be set per box.

This also fixes boxplot when range is zero. Before this PR nothing would get plotted (because the color vector was empty), after this PR the plot works correctly with zero range.

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

Checklist

• [x] Added an entry in CHANGELOG.md (for new features and breaking changes)
• [ ] Added or changed relevant sections in the documentation
• [ ] Added unit tests for new algorithms, conversion methods, etc.
• [ ] Added reference image tests for new plotting functions, recipes, visual options, etc.