Add new models

While allowing for the use of other models than GR5J is not a priority (but seems not that difficult), I would encourage you to allow for the use of CemaNeigeGR5J. @gaia.piazzi , I guess this would work and we don't necessarily need to perturb temperature, what do you think? What about using '...' to give the necessary arguments of CreateInputsModel for snow model?

In addition, I propose replacing this error message in CreateInputsPerturb:

stop("incorrect 'FUN_MOD' for use in 'MeteoPert'. Only 'RunModel_GR5J' can be used")

with

stop("incorrect 'FUN_MOD' for use in 'CreateInputsPerturb'. Only 'RunModel_GR5J' can be used")

Same in Run_ModelDA. Replace:

stop("incorrect 'FUN_MOD' for use in 'MeteoPert'. Only 'RunModel_GR5J' can be used")

with

stop("incorrect 'FUN_MOD' for use in 'RunModel_DA'. Only 'RunModel_GR5J' can be used")