Deprecate object composition

[nfahlgren]

Describe your changes
Deprecates the pcv.object_composition function. Refactors pct.report_size_marker to utilize the helper function _object_composition instead of the public function.

Type of update
Is this a: update

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 #1216 (27d71f1) into release-4.0 (c103c33) will not change coverage.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           release-4.0     #1216   +/-   ##
=============================================
  Coverage       100.00%   100.00%           
=============================================
  Files              162       161    -1     
  Lines             6987      6957   -30     
=============================================
- Hits              6987      6957   -30     

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/__init__.py	100.00% <ø> (ø)
plantcv/plantcv/report_size_marker_area.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes