Add helper function to centralize usage of OpenCV findContours

[nfahlgren]

Describe your changes
We utilize cv2.findContours throughout PlantCV and use a mixture of directly using the OpenCV function or use the PlantCV wrapper find_objects. In most cases it's simpler to use the OpenCV function rather than using the PlantCV wrapper within the code. Additionally, if OpenCV ever changes the function signature then PlantCV has to be changed in many places. This PR adds a helper function that calls cv2.findContours and replaces direct usage throughout PlantCV to use the helper function.

Type of update
Is this a: Code refactoring

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

[codecov]

Codecov Report

Merging #939 (92f29cb) into 4.x (e836702) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 92f29cb differs from pull request most recent head 50e22f0. Consider uploading reports for the commit 50e22f0 to get more accurate results

@@            Coverage Diff            @@
##               4.x      #939   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files          160       161    +1     
  Lines         6827      6840   +13     
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+ Hits          6827      6840   +13     

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

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

Impacted Files	Coverage Δ
plantcv/plantcv/_helpers.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze_bound_horizontal.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze_bound_vertical.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze_object.py	100.00% <100.00%> (ø)
plantcv/plantcv/crop_position_mask.py	100.00% <100.00%> (ø)
plantcv/plantcv/find_objects.py	100.00% <100.00%> (ø)
plantcv/plantcv/morphology/_iterative_prune.py	100.00% <100.00%> (ø)
plantcv/plantcv/morphology/check_cycles.py	100.00% <100.00%> (ø)
plantcv/plantcv/morphology/find_branch_pts.py	100.00% <100.00%> (ø)
plantcv/plantcv/morphology/find_tips.py	100.00% <100.00%> (ø)
... and 11 more