Refactor covariance matrix production rule and rename explicit node

[comane]

This removes the need for the use_gen_t0 key in the runcard.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.00%. Comparing base (cf4d16e) to head (21cb95f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #432      +/-   ##
==========================================
+ Coverage   94.96%   95.00%   +0.03%     
==========================================
  Files          31       31              
  Lines        1649     1642       -7     
==========================================
- Hits         1566     1560       -6     
+ Misses         83       82       -1     

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[comane]

Analytical L1 fit fails because the runcard does not have the use_gen_t0 set to True.

[LucaMantani]

Why did you decide to remove it? What was the reason for us to have the different covmats before?

[comane]

Why did you decide to remove it? What was the reason for us to have the different covmats before?

The reason was having the possibility of generating pseudodata (in the MC replica method) with the experimental covariance matrix rather than the t0-modified covariance matrix that we use in the fit.

However, the NNPDF standard now is to use also t0-covmat in the pseudodata generation.