Add radial percentile function

[k034b363]

Describe your changes
This PR adds a function pcv.analyze.radial_percentile that calculates the average pixel values within a threshold distance from the center of a focal object, expressed as a percent of maximum distance. It also adds this calculated average to outputs. It can take as input either a grayscale or RGB image, in which case the number of outputs will match the number of channels. It can also take an ROI or list of ROIs to calculate the average for multiple objects.

Type of update

• New feature or feature enhancement

Associated issues
Closes #1646 (in a simpler way than described in that issue)

Additional context
I am intending to use this function as is for a paper on seed Xray analysis, so the decision to simplify was in service of movement on that project. There are options to make this function more complex in the future if use cases are discovered.

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 8f2b592..bcaa177. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		View Check ↗
Test coverage	⚠️ Artifact not reported	Timed out: Artifact was never reported	View Check ↗

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