Enable the GR models to ingest ensembles
Ensembles are useful for data assimilation and probabilistic forecasting applications. It might be useful to add the possibility to give ensembles of meteorological forcing to the GR models. Presently ensembles are dealt with through loops on each member, but providing directly the ensembles could fasten the execution of the GR models (indeed, the present loops necessitate to relaunch the GR models at each time step).