Enable multi label input

[nfahlgren]

Describe your changes
Modifies the helper function _iterate_analysis to accept either a string label or a list of labels equal to the length of n_labels. If a string is input, a list of length n_labels is made by repeating the input string. The iterator uses the list of labels to label each object label. If the length of labels is not equal to n_labels a fatal error is triggered.

We might need to update all analysis doc pages to indicate that a list of labels can be input.

Type of update
Is this a:

• New feature or feature enhancement

Associated issues
#1360

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 #1361 (6d6e701) into main (5fe40c7) will not change coverage.
The diff coverage is 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main     #1361   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          155       155           
  Lines         6840      6861   +21     
=========================================
+ Hits          6840      6861   +21     

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

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

Files	Coverage Δ
plantcv/plantcv/_helpers.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/npq.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/yii.py	100.00% <100.00%> (ø)

[maliagehan]

Checked in a multiplant workflow and checked output.