Add ``figures`` property to ``DatasetPlot``

Problem to solve

The standard approach to use DatasetPlot in plotting methods in GEMSEO (SensitivityAnalysis.plot, MLQualityViewer.plot is to return the plot object. The plot is executed in the plotting method. Since the figures generated by the execute method of the plots is currrently not accessible, the user has to execute the plot again after retrieval from the GEMSEO plot service. But he has already passed arguments to define how to execute the plot, so it is weird that he has to pass again these arguments to re-execute the plot (though this can be a useful scenario if he wants to customize the plot, or add another trace on the plot).

Proposal

A property figure could be added to the DatasetPlot, filled in the execute method of the plot. As such, the user can directly retrieve the generated figures.