Fix for Merge_tf.py in AthDerivation

The python modules in SimuJobTransforms are not available in
AthDerivation, and Merge_tf crashes while attempting to import
them for its HITSMerge substep. This branch makes the import
of these modules conditional to their availability in the release


Former-commit-id: 4557e6035658e3db7f09ef700a837936a91f7a03