PyJobTransforms: Add multithreadedFileValidation option
As discussed in ATLASJT-424, add the new option --multithreadedFileValidation to the PyJobTransform package. This option enables multithreaded ROOT in the output file validation step of e.g. DAOD_PHYS/LITE. In the communication with the actual file validation script trfValidateRootFile.py an environment variable is used, since it seems impossible to pass the --multithreadedFileValidation info through the full object oriented PyJobTransforms framework otherwise.