Add warn cluster contour splitimg

[zeeuqsze]

Describe your changes
Added warn function, replacing print statements.

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 #1160 (9c07588) into release-4.0 (df9b956) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1160   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              167       167           
  Lines             7270      7268    -2     
=============================================
- Hits              7270      7268    -2     

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/cluster_contour_splitimg.py	100.00% <100.00%> (ø)