Add segment_ends and Update morphology tips outputs

[HaleySchuhl]

Describe your changes

• Moves _iterative_prune into the _helper.py file to avoid circular imports (this unfortunately touches most of the functions in this package)
• update find_tips (adds a helper function _find_tips that is used internally by many functions and doesn't store outputs so they do not accidentally get overwritten)
• adds pcv.morphology.segment_ends which returns: Re-ordered leaf segments, debug image, list of branch point coordinates, list of leaf tip coordinates
• bugfix to pcv.morphology.analyze_stem where the label=None was causing no data to get saved to Outputs. Resolved by updating with appropriate labels.

Type of update
Is this a:

• Bug fix
• New feature or feature enhancement

Associated issues

• closes Skeleton tips easily overwritten within the plantcv.morphology sub-package #1643

Additional context

• Using the internal segment end coordinates for leaf ID order
• The saved skeleton tips coordinates to Outputs was easily overwritten by other morphology functions that utilize the pcv.morphology.find_tips function.
• the deepsource complaint about the complexity of segment_insertion_angle will be resolved later in another PR branch (called "update_segment_insertion_angle")
  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

[deepsource-io]

Here's the code health analysis summary for commits f4caba5..5b45790. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	❌ Failure	❗ 1 occurence introduced 🎯 1 occurence resolved	View Check ↗
Test coverage	✅ Success		View Check ↗

Code Coverage Report

Metric	Aggregate	Python
Branch Coverage	100%	100%
Composite Coverage	100%	100%
Line Coverage	100%	100%
New Branch Coverage	100%	100%
New Composite Coverage	100%	100%
New Line Coverage	100%, ✅ Above Threshold	100%, ✅ Above Threshold

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[k034b363]

Looks good to me, it works the way I would expect it to and reorders branches to something biologically meaningful on both well behaved and less clean maize pictures. The movement of some functions to helpers makes the previously complex morphology functions quite a bit easier to read as well.