Group covariance matrices should take into account correlations

Created by: Zaharid

They are now implemented using collect over datasets:

https://github.com/NNPDF/nnpdf/blob/97ee037483839210a607713f0a9e572eb4241ee2/validphys2/src/validphys/results.py#L241

but in principle the correlations between datasets should be taken into account.