Migrating PDF4LHC15->21

Matthew Gignacrequested to merge
mgignac/athena:sherpa_i-add-pdf4lhc21 into master

Migrating PDF4LHC15->21 in Sherpa_i, which have become available as of yesterday. Adding the central values of a few more: NNPDF40_nnlo_as_01180_hessian CT18ANNLO CT18XNNLO CT18ZNNLO

Tagging @fsiegert @jkretz

Edited by Matthew Gignac

Merge request reports