Generic hessian PDF covmat
Created by: comane
This PR adds to covmats.pdferr_plus_covmat
the option for an asymmetric hessian set.
The way in which the PDF covmat is computed follows core.HessianStats
.
TODO:
- Are there some references on how to compute the hessian pdf covmat?
- understand why the ABMP1_6_5_nnlo set (symmhessian) has such a large chi2 (see report below)