This MR updates the default PDFs used in MG, following the recommendations for Run 3. We keep NNPDF3.0 as the nominal PDF but switch to Hessian errors. We switch from PDF4LHC15NLO to PDF4LHC21(NNLO) for the alternative error set. A number of newer versions of PDFs (e.g. NNPDF3.1 and 4.0) are added (central PDF only). We now add both NLO and NNLO versions of PDFs.
Partially, this was already implemented but now minor issues are fixed and alternative settings (e.g. nf4) are updated, too.