Writing parquet files with newer datetime format

[nmerket]

Fixes #221.

Pull Request Description

Removes the flavor='spark' from postprocessing.py as it no longer does what was intended and isn't necessary given Athena can understand the new datetime format.

I have run a small test with this and confirmed that Athena properly interprets the datetime fields.

Checklist

Not all may apply

• Code changes (must work)
• Tests exercising your feature/bug fix (check coverage report on CircleCI build -> Artifacts)
• All other unit tests passing
• Update validation for project config yaml file changes
• Update existing documentation
• Run a small batch run to make sure it all works (local is fine, unless an Eagle specific feature)
• Add to the changelog_dev.rst file and propose migration text in the pull request