vendor bits of openmm code to drop full openmm dependency

[atravitz]

resolves #662

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Checklist

• Added a news entry

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• I certify that this contribution is covered by the MIT License here and the Developer Certificate of Origin at https://developercertificate.org/.

[github-actions]

No API break detected ✅

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.88%. Comparing base (ceeda21) to head (33f8f3f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #740   +/-   ##
=======================================
  Coverage   98.88%   98.88%           
=======================================
  Files          42       42           
  Lines        2685     2686    +1     
=======================================
+ Hits         2655     2656    +1     
  Misses         30       30           

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