Use S3 plot method for GRiwrm and GRiwrmOutputsModel class objects

• replace DiagramGRiwrm by plot.GRiwrm
• Add class GRiwrmOutputsModel in the list returned by RunModel.GRiwrmInputsModel
• Add id item in OutputsModel embedded in GRiwrmOutputsModel
• Create the method plot.GRiwrmOutputsModel with the following code:

lapply(
  x,
  function(y) {
    plot(y, Qobs = Qobs[,y$id] , main = y$id, ...)
  }
)