Move parallel module outside utils

[mdbenito]

Description

This PR refactors the parallel module. We should probably do the same with half of utils

Changes

• moves utils.parallel out of utils
• moves ParallelConfig into pydvl.parallel.config.
• importing from pydvl.utils and from pydvl.utils.parallel still works and displays a deprecation warning. If we merge this I'll create an issue to remove the old module on that release

Checklist

• Wrote Unit tests (if necessary)
• Updated Documentation (if necessary)
• Updated Changelog
• If notebooks were added/changed, added boilerplate cells are tagged with "tags": ["hide"] or "tags": ["hide-input"]