Adding draft GENtoEVGEN_Skeleton module and cleanup of unused arguments for Gen_tf

• Removed following unused arguments from Gen_tf. If the user provides these Gen_tf.py will immediately crash saying that these arguments are not recognised
  • --inputGenConfFile
  • --runNumber
• Added skeletonCA options in evgenExecutor
• Added dummy file for GENtoEVGEN_Skeleton.py that will be later populated (see !67233 (closed))

Tested on 421007 that runs on the CI also the new Gen_tf.py --CA option and it runs

cc @ewelina @tadej

PS: we can also take this opportunity to maybe do a bit of cleanup including some stylistic fixes in Gen_tf.py? (or in a separate MR)