source visualization validation fix

[tontyoutoure]

fix two minor problems:

1. MT mode will cause source visualization position error, so I made it to generate a fatal error to notify the user
2. if sim.visu = False and in MT mode and some of the sources are set visualization.count = 0, then

logger.info(
                f"For source {parent_obj.name}, visualization count is set to {b.count}. No visualization will be performed."
            )

will be printed by each of the thread source, which is annoying.