RunModel_Lag: Add an argument `QcontribDown` to `RunModel_Lag`

Currently, the runoff contribution of the downstream sub-basin is included in the InputsModelobject by copying the OutputsModel object of the GR simulation (extracted from RunModel_lag example script):

# Add the output of the precipitation-runoff model in the InputsModel object
InputsModel$OutputsModel <- OutputsModelDown

# Run the lag model which routes precipitation-runoff model and upstream flows
OutputsModel <- RunModel_Lag(InputsModel = InputsModel,
                             RunOptions = RunOptions, Param = Velocity)

This way of proceeding is:

• ugly 😅
• not generic if you want to inject another contribution than a GR model output
• cumbersome because we don't need to copy all informations contained in OutputsModel, only Qsim

Using a new argument QcontribDown in RunModel_Lag for the runoff contribution of the downstream sub-basin would be cleaner and more generic.