Enable Parsl app concurrency with futures

[d33bs]

Description

This PR addresses an issue with Parsl app evaluation to enable concurrency throughout _to_parquet() and related. It stems from investigation through #192, originally inspired by work towards documenting #176 (after finding no significant differences in Parsl configurations performance). After making these changes mypy was upset about a few type hints, so I updated these as well to ensure pre-commit checks could pass.

Generally, this work focused on following the patterns mentioned under Parsl documentation on using Futures.

Closes #192

What is the nature of your change?

• [x] Bug fix (fixes an issue).
• [ ] Enhancement (adds functionality).
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected).
• [ ] This change requires a documentation update.

Checklist

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• [x] I have made corresponding changes to the documentation.
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[d33bs]

Thank you @kenibrewer ! Merging this in.