bugfix for roi_type="largest" in roi.filter

[HaleySchuhl]

Describe your changes
Updates the way we determine which contour is largest after filtering in pcv.roi.filter with roi_type="largest" setting. Merge this AFTER #1520

Type of update
Is this a:

• Bug fix
• Work in progress

Associated issues

• closes Bug in logic behind roi_type="largest" option inside pcv.roi.filter  #1507

Additional context
Add any other context about the problem here.

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• PR functionality reviewed in a Jupyter Notebook
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• Documentation tested
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[deepsource-io]

Here's the code health analysis summary for commits de60aec..b86af6b. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success	🎯 2 occurences resolved	View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	99.7%	99.7%
Line Coverage	99.7%	99.7%
New Branch Coverage	100%	100%
New Composite Coverage	100%	100%
New Line Coverage	100%, ✅ Above Threshold	100%, ✅ Above Threshold

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[HaleySchuhl]

Added different assertions for the unit test. Instead of saying the contour is exactly xx pixels, we can make sure that the contour size is the same as before filtering out the small object. Also added a small object to the "pre" mask so that the function is actually doing something.