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 saves both ends, the tip coordinates and the coordinates of the segment touching the branch point.

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 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 432fcd2..a38e353. 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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[HaleySchuhl]

Jan 3 updates: the deepsource complaint about the complexity of segment_insertion_angle will be resolved in another PR branch (called "update_segment_insertion_angle").

While thinking about how to get biologically-ordered observations out from a morphology workflow given this new function, there is a reason to return a re-ordered list of objects. These can then get used downstream to get consistently labeled leaf segment observations.

[k034b363]

It looks like the context is incomplete, just needs the end of that sentence about grass species.

[k034b363]

Also, in the example usage there is a parenthesis missing, and I think the two segment_id examples are reversed? Should the "without id reassignment" example be with leaf_objs, not sorted_obs?