feat: (Re-)Enable MPI for pMSSM
See ATLMCPROD-9780 for the discussion. @mgignac found this to be disabled which was done in Run 1 -- and just kind of copied over. It was determined in initial studies that there was no impact in efficiency, but probably will have more convoluted/complicated impacts on the actual physics for something large-scale like pMSSM... so re-enabling as it's not necessarily a chokehold for the generation anymore.