Sankey diagram: add option to see sankey for 1 elementary flow

[CHarpprecht]

Hi ;) I think it would be helpful for the Sankey Diagram to see the diagram not only per impact category but also for individual biosphere flows. Such that there is a third drop down menu below the "impact indicator" where one could select one biosphere flows from the selected impact category. This helps e.g. to identify the source of a certain biosphere flow, which was found to be interesting during the EF contribution analysis. I might be wrong here, but so far I do not see an easy way to find out which process is the one causing high emissions of a certain biosphere flow.

[BenPortner]

Alternative suggestion: Show biosphere flows as nodes in the sankey diagram, just like technosphere activities. This would allow tracing all impacts down to the individual biosphere flow, rather than staying on the activity level.