Fix mask kmeans output type

[k034b363]

Describe your changes
The outputs of both kmeans_classifier functions (predict and mask) were types other than uint8. This PR fixes both functions so that the output types are consistent with downstream PlantCV analysis

Type of update
Is this a:

• Bug fix

Associated issues
Reference associated issue numbers. Does this pull request close any issues?

Additional context
Add any other context about the problem here.

For the reviewer
See this page for instructions on how to review the pull request.

• PR functionality reviewed in a Jupyter Notebook
• All tests pass
• Test coverage remains 100%
• Documentation tested
• New documentation pages added to plantcv/mkdocs.yml
• Changes to function input/output signatures added to updating.md
• Code reviewed
• PR approved

[deepsource-io]

Here's the code health analysis summary for commits c6105a7..5800332. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%
New Branch Coverage	100%	100%
New Composite Coverage	100%	100%
New Line Coverage	100%, ✅ Above Threshold	100%, ✅ Above Threshold

---

💡 If you’re a repository administrator, you can configure the quality gates from the settings.