Refactor parallel.py (NGWPC-10583)

[idtodd]

Rework parallel.py to add appropriate documentation and clean the processing.

Primary changes include remove the need to pass a ConfigOptions object to the methods since an instance of ConfigOptions will be saved on __init__.

Additionally, the cleanup is no longer being tied to atexit and instead needs to be explicitly called when exiting the program. The workflow of NGEN will almost always call the BMI's finalize method, so including cleanup in there should let normal runs of NGEN continue cleaning temporary files properly whilst removing the messy atexit dependency.

Additions

Removals

Changes

Testing

Screenshots

Notes

Todos

Checklist

• PR has an informative and human-readable title
• Changes are limited to a single goal (no scope creep)
• Code can be automatically merged (no conflicts)
• Code follows project standards (link if applicable)
• Passes all existing automated tests
• Any change in functionality is tested
• New functions are documented (with a description, list of inputs, and expected output)
• Placeholder code is flagged / future todos are captured in comments
• Visually tested in supported browsers and devices (see checklist below 👇)
• Project documentation has been updated (including the "Unreleased" section of the CHANGELOG)
• Reviewers requested with the Reviewers tool ➡️

Testing checklist

Target Environment support

• Linux