Refactor analytical fit to use QR decomposition and remove central covariance

[comane]

What this PR does:

• removes the central_inv_covmat_index object. The object was not really necessary and only used within the analytical fit
• Refactors the analytical fit so as to efficiently make use of QR decomposition in solving the GLS.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.96%. Comparing base (74b9b9f) to head (2b2efe2).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #433      +/-   ##
==========================================
- Coverage   94.98%   94.96%   -0.02%     
==========================================
  Files          31       31              
  Lines        1654     1649       -5     
==========================================
- Hits         1571     1566       -5     
  Misses         83       83              

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[LucaMantani]

Looks good to me!