Deprecate cluster contours mask

[HaleySchuhl]

Describe your changes
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What was the previous functionality (if relevant) and what can we do now with
these changes.

Type of update
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• Bug fix
• New feature or feature enhancement
• Update to documentation
• Work in progress

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For the reviewer
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• 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

[codecov]

Codecov Report

Merging #1197 (0f64e10) into release-4.0 (e4c25b8) will not change coverage.
The diff coverage is n/a.

@@              Coverage Diff              @@
##           release-4.0     #1197   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              172       171    -1     
  Lines             7530      7528    -2     
=============================================
- Hits              7530      7528    -2     

Flag	Coverage Δ
unittests	100.00% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
plantcv/plantcv/__init__.py	100.00% <ø> (ø)

... and 1 file with indirect coverage changes