update roi_objects with Objects class

[JorgeGtz]

Describe your changes
Modify roi_objects to use instances of the Objects class instead of contours and hierarchies.
In this first step the function will only work with single ROIs. If provided with a multi-ROI Objects instance it shows a warning and only uses the first element. In order to use a multi-ROI Object, the user should iterate.

Type of update
Is this a:

• New feature or feature enhancement

Associated issues
#945

Additional context

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 #1037 (c9edebc) into release-4.0 (662f488) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1037   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              164       164           
  Lines             7004      7015   +11     
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+ Hits              7004      7015   +11     

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/report_size_marker_area.py	100.00% <100.00%> (ø)
plantcv/plantcv/roi_objects.py	100.00% <100.00%> (ø)