Get `plot.Qm3s` available as a function

The method plot for the class "Qm3s" is useful for plotting flow rate time series outside of the result of the simulated flows provided by attr(GRiwrmOutputsModel, "Qm3s").

Currently the call of plot.Qm3s raises an error : Error in plot.Qm3s(dfQdiv) : could not find function "plot.Qm3s"

Tasks:

• add checks
• complete the doc
• modify exported object in NAMESPACE (https://stackoverflow.com/questions/18512528/how-to-export-s3-method-so-it-is-available-in-namespace)