Powheg script runcmsgrid_powheg.sh separate the index for different PDF set errors

A very minor change but hope to make the PDF weights more easily to understand.
Give the weights of each PDF set (NNPDF30, CT10, CT10alphas, MSTW, NNPDF23) different ID.

The output looks like this LHE file:
/afs/cern.ch/work/s/syu/public/LHE/cmsgrid_final.lhe