refactor openmp task-based internal use + add policy interface to factorization

[PierreMarchand20]

No description provided.

[codecov]

Codecov Report

❌ Patch coverage is 98.63014% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.03%. Comparing base (658d8f0) to head (e292614).
⚠️ Report is 53 commits behind head on main.

Files with missing lines	Patch %	Lines
...ed/test_task_based_hmatrix_full_complex_double.cpp	92.30%	1 Missing ⚠️
...task_based/test_task_based_hmatrix_full_double.cpp	92.30%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #86      +/-   ##
==========================================
+ Coverage   97.19%   98.03%   +0.83%     
==========================================
  Files          80      185     +105     
  Lines        5132     9951    +4819     
==========================================
+ Hits         4988     9755    +4767     
- Misses        144      196      +52     

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