Remove useless time step checks in the plot.OutputsModel function

The time step of the OutputsModel argument is checked by the plot.OutputsModel() function (the time step is checked by comparing with the calculation of the difference of the last two time steps). This seems useless, because we already check the class of this object, which is therefore assumed to be necessarily valid.

In addition, this stops the function from being used on an aggregated series returned by the SeriesAggreg2.OutputsModel () function.