clarify management of gemoc-gen folder

currently when running multiple simulations, the engine creates one folder per execution in the gemoc-gen folder.

most of the time this only contains a copy of the executed model without any additional interesting data. In the past these folders were used to also store some trace model

we should either:
- avoid its creation when not required
- or automatically clean it (especially when these folders don't contain any valuable data)
- or make sure to always overite the same folder (with the question of the possible simultaneous run of the same model with possibly different arguments ?)

Note: the exact behaviour may be different between engines which may create additional files there