Merge Certification:
The second dataset is different for each site and will be used to certify the merging step
by comparing the unmerged with the merged plots at each site.
It checks that the merging procedure doesn't modify the data.
Merge certification is done centrally for each release to check the tarballs used for merging.
It done only once per remote site (sites that participated in p17 reprocessing may thus skip it).