Add analyze color function to analyze submodule

[nfahlgren]

Describe your changes
Moves the analyze_color function to pcv.analyze.color. Refactors the code to use the _iterate_analysis helper function. Supports labeled masks with automatic iteration over multiple input objects.

Also adds a built-in distribution plotting method to the Outputs class that is used by pcv.analyze.color and other analysis functions that plot histograms.

Type of update
Is this a: New feature or feature enhancement

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 #1170 (fc8e740) into release-4.0 (e0d7c30) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1170   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              167       168    +1     
  Lines             7268      7351   +83     
=============================================
+ Hits              7268      7351   +83     

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/analyze/__init__.py	100.00% <100.00%> (ø)
plantcv/plantcv/analyze/color.py	100.00% <100.00%> (ø)
plantcv/plantcv/classes.py	100.00% <100.00%> (ø)
plantcv/plantcv/plot_image.py	100.00% <100.00%> (ø)
plantcv/plantcv/print_image.py	100.00% <100.00%> (ø)