Replace OMP reductions

[marvinfriede]

See also grimme-lab/mctc-lib#79 and grimme-lab/mctc-lib#81.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 66.55%. Comparing base (61714e3) to head (5f7f5ad).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #137      +/-   ##
==========================================
- Coverage   66.75%   66.55%   -0.21%     
==========================================
  Files          33       33              
  Lines        4666     4682      +16     
  Branches     1646     1661      +15     
==========================================
+ Hits         3115     3116       +1     
  Misses        636      636              
- Partials      915      930      +15     

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