Update roi rectangle objects class

[HaleySchuhl]

Describe your changes
Update pcv.roi.rectangle to utilize the Objects class.

Type of update
Is this a:

• New feature or feature enhancement
• Update to documentation

Associated issues

• Related to Release 4.0 - Update code base to utilize Objects class  #945

Additional context
Add any other context about the problem here.

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 #946 (ed30fd1) into release-4.0 (4a514f7) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0      #946   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              161       161           
  Lines             6840      6841    +1     
=============================================
+ Hits              6840      6841    +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]

Perfect, thanks!