The graphs linked to a simulation should be saved into the result datapackage structure

[Bachibouzouk]

Once a graph is produced using eesyplan's fonctions one should be able to store it in order to reload the result page quickly.

To do so we could save the fig.to_dict() (or to_json()) within the result datapackage's structure

results
    elements/
    sequences/
    figures/
    datapackage.json

The aim is that be reading the datapackage one can regenerate the plotly figures or that one can simply go through the figures/ folder and regenerate the figures.

We could start a prototype of this with the sankey graph, as it is almost implemented within #479.