changing parallelization defaults

[joshqsumner]

Describe your changes
Changes to default settings in plantcv/plantcv/parallel/__init__.py based on issue #1639

Type of update
Is this a feature enhancement based on common parallelization questions.

Associated issues
Closes #1639

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 53c783b..03d2fb2. 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%, ✅ Above Threshold	100%, ✅ Above Threshold

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[HaleySchuhl]

Used this branch to make a template config file and looks good to me. I updated one more doc page to reflect the new defaults.