use error_members instead of rawdata
Created by: scarlehoff
This is straightforward for MC PDFs (they are the same thing now and this change will make the change from all_members to only_replicas very simple) but for Hessian PDFs is instead very dangerous since the error members are the rawdata-1.
Making this as a separate PR since it makes many changes in the code but are almost a sed
(a manual one nonetheless) and because I will be running the fit bot quite a bit I guess.