[SimWF] Protect the -e argument (#1551)

Without this, passing -embedding would work and produce a workflow.json.
However, the resulting WF has nothing to with embedding, since the real
flag for that has 2 dashed: --embedding
What really happened was that the engine would now be "mbedding"

So far there are no clashes for -j or -o which are also valid arguments.

Co-authored-by: Benedikt Volkel <[email protected]>

MC/bin/o2dpg_sim_workflow.py

@@ -85,7 +85,7 @@
 parser.add_argument('-confKeyBkg',help='embedding background configuration key values, for example: "GeneratorPythia8.config=pythia8bkg.cfg"', default='')
 parser.add_argument('-colBkg',help='embedding background collision system', default='PbPb')
 
-parser.add_argument('-e',help='simengine', default='TGeant4')
+parser.add_argument('-e',help='simengine', default='TGeant4', choices=['TGeant4', 'TGeant3', 'TFluka'])
 parser.add_argument('-tf',help='number of timeframes', default=2)
 parser.add_argument('--production-offset',help='Offset determining bunch-crossing '
                      + ' range within a (GRID) production. This number sets first orbit to '