make Objects class return Objects instance while iterating

[JorgeGtz]

Describe your changes
A clear and concise description of what changes are made by this pull request.
What was the previous functionality (if relevant) and what can we do now with
these changes.

Type of update
Is this a:

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

Associated issues
Reference associated issue numbers. Does this pull request close any issues?

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 #1068 (469f2e5) into release-4.0 (912c400) will not change coverage.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##           release-4.0    #1068   +/-   ##
============================================
  Coverage        99.98%   99.98%           
============================================
  Files              161      161           
  Lines             6851     6851           
============================================
  Hits              6850     6850           
  Misses               1        1           

Flag	Coverage Δ
unittests	99.98% <100.00%> (ø)

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

Impacted Files	Coverage Δ
plantcv/plantcv/classes.py	100.00% <100.00%> (ø)
plantcv/plantcv/roi/roi2mask.py	100.00% <100.00%> (ø)