Update roi custom objects class

[HaleySchuhl]

Describe your changes
pcv.roi.custom

Associated issues
#945

Additional context
Add any other context about the problem here.

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 #953 (e42fe7a) into release-4.0 (99041b4) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0      #953   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              161       161           
  Lines             6842      6843    +1     
=============================================
+ Hits              6842      6843    +1     

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/roi/roi_methods.py	100.00% <100.00%> (ø)

[nfahlgren]

Thanks!