add basic functionality for annotating Points

[HaleySchuhl]

Describe your changes
Add "point" annotation, ability to import and export (save out) collected point annotations, and plotting. Also ability to label different classes of annotations (e.g. pollenated vs un-pollenated) and save class counts.

Code originally added to this old branch danforthcenter/plantcv#754 but the PR became unruly and difficult to push forward with changes without breaking changes, so separated out to it's own repository here.

Type of update
Is this a:

• New feature or feature enhancement
• Work in progress

Associated issues
Reference associated issue numbers. Does this pull request close any issues?

Additional context

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 cfb9d5f..8ff04cc. View details on DeepSource ↗.

Analysis Summary

Analyzer	Status	Summary	Link
Python	✅ Success		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%	100%

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[HaleySchuhl]

Currently, we have named the method to print text file .print_coords, and intend to have another method called .save_coords that talks to pcv.Outputs class (after the repo has been moved out and above the main plantcv repo). Names were selected based on other PlantCV functions and methods (print to file and save to outputs) but open to other ideas. We are allowing ourselves to make breaking changes as we see fit before doing a real conda version release.

[nfahlgren]

We will add documentation later