Update sample_images to handle phenodata datasets

[nfahlgren]

Describe your changes
plantcv.utils.sample_images (used by plantcv-utils.py sample_images) was not updated to handle phenodata datasets (see #923). This PR refactors sample_images to modularize the code and adds a sampling method for phenodata datasets.

Type of update
Is this a: Bug fix

Associated issues
Closes #923

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 #925 (3decc4b) into 4.x (4e1bf48) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##               4.x      #925   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          160       160           
  Lines         6799      6826   +27     
=========================================
+ Hits          6799      6826   +27     

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/utils/sample_images.py	100.00% <100.00%> (ø)