`plot.GRiwrmOutputsModel`: handle other units than mm / time step

The method airGR::plot.OutputsModel handles results in m3/s if the parameter BasinArea is provided (km²). The idea is to be able to use this functionnality through plot.GRiwrmOutputsModel.

TODO:

• embed the GRiwrm object in the attributes of GRiwrmOutputsModel
• add a parameter unit which accepts "mm" and "m3/s" to plot.GRiwrmOutputsModel (default "m3/s")
• use the GRiwrm object to get area of each basin and pass it to each call of plot.OutputsModel if unit "m3/s".