Add OpenMP option for CMake

[awvwgk]

Closes #57

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69.79%. Comparing base (77f65c6) to head (16ce44d).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #65   +/-   ##
=======================================
  Coverage   69.79%   69.79%           
=======================================
  Files          64       64           
  Lines        8618     8618           
  Branches     2579     2579           
=======================================
  Hits         6015     6015           
- Misses        782      783    +1     
+ Partials     1821     1820    -1     

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