Replace donut plots with stacked bar graph

[standage]

Currently, a breakdown of reads by category (on target, off target, repetitive, and contamination) is displayed in the report as a grid of donut plots, one per sample. This PR replaces these donut plots with a single stacked bar chart. It is shown in horizontal orientation so that the height of the plot can grow as the number of samples grows.

Related to #160.

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• [x] Changes are clearly described above
• [x] Any relevant issue threads are referenced in the description
• [x] Any new features are tested (see the development manual for details)
• [x] CLI documentation (see docs/cli.md) and Python API documentation (see microhapulator/api.py) are up-to-date and in sync
• [x] Substantial changes are documented in CHANGELOG.md (see https://keepachangelog.com/en/1.0.0/)

[standage]

This is how the plot looks in context.

[agshumate]

Code looks good but legend placement looks a little funky with a small number of samples

[standage]

An additional line of code seems to have addressed this issue, and appears to work as intended regardless of the number of samples.

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[agshumate]

Looks good!