add support for multiProcesses cmsRun to the DQMFileSaver

Minimal patch to add support for "multiProcesses cmsRun" (i.e., forking children process from a cmsRun master) to the DQMFileSaver.

It adds _N to the the file names used by the DQM, using the ame logic as the PoolOutputModule.

The DQM files produced in this way should then be merged together to recover the original file name before begin loaded in the DQM GUI.