Reactivate THREADS keyword

[godotalgorithm]

As noted in #37, the THREADS keyword was previously deactivated because it relied on MKL-specific functions, and development of MOPAC has moved to more general support of BLAS & LAPACK libraries. However, all known threaded BLAS/LAPACK libraries are based on OpenMP, so we can again support the THREADS keyword through generic OpenMP API calls. The MKL-specific functionality has been hidden behind the MKL preprocessor variable for a while, which we are keeping in until the dense linear algebra refactor happens. There is some redundancy between the new OMP_ENABLED-filtered code blocks and the deprecated MKL-filtered code blocks, but it is practically irrelevant.

Status

• [x] Ready for merge

[codecov-commenter]

Codecov Report

Merging #55 (6d07c0b) into main (f230b06) will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #55   +/-   ##
=======================================
  Coverage   68.22%   68.22%           
=======================================
  Files         330      330           
  Lines       71684    71690    +6     
=======================================
+ Hits        48906    48912    +6     
  Misses      22778    22778           

Impacted Files	Coverage Δ
src/run_mopac.F90	78.21% <100.00%> (+0.20%)	:arrow_up:

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