Allow to use node with model RunModel_Lag

The easiest way to simulate nodes that represents canal outlets without impluvium should be to use a model RunModel_Lag to only transfer flows from upstream nodes without taking into account potential run-off.

Unfortunately using RunModel_Lag as a model, raises lot of issues which seems to be related to malfunctions on both airGRiwrm and airGR...

For example the following code based on the data available in the vignette Severn_01 crashes:

db <- data.frame(id = c("54095", "DownLag"),
                   length = c(1, NA),
                   down = c("DownLag", NA),
                   area = as.double(c(3722.68, NA)),
                   model = c("RunModel_GR4J", "RunModel_Lag"),
                   stringsAsFactors = FALSE)
  g <- CreateGRiwrm(db)
  IM <- CreateInputsModel(g,
                          DatesR = DatesR,
                          Precip = Precip[, "54095", drop = FALSE],
                          PotEvap = PotEvap[, "54095", drop = FALSE])

CreateInputsModel.GRiwrm: Processing sub-basin 54095...
CreateInputsModel.GRiwrm: Processing sub-basin DownLag...
Error in if (BasinAreas[length(BasinAreas)] < 0) { : 
  missing value where TRUE/FALSE needed