Add schema for atomate2 classical_md workflows

[orionarcher]

This adds in schema for the nearly complete atomate2.classical workflows in this PR.

The PR is just pydantic schema and no new functionality or dependencies are added.

Contributor Checklist

• [x] DONE: add atomate2 classical_md schema
• [x] I have run the tests locally and they passed.
• [x] I have added tests, or extended existing tests, to cover any new features or bugs fixed in this PR

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 90.01%. Comparing base (5cc66a2) to head (dffb92c). Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #975 +/- ## ======================================= Coverage 90.01% 90.01% ======================================= Files 138 138 Lines 13226 13226 ======================================= Hits 11906 11906 Misses 1320 1320 ```

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[orionarcher]

Thanks @rkingsbury! I'll integrate this feedback into my next PR.