calculate low carbon energy when using energy_date_selection

[macmeck]

When using the Sankey Chart to display historic energy values which are selected by energy_date_selection, we have no easy way to display the low carbon energy part from the total energy. Since the carbon intensity varies depending on the timeframe selected, a static calculation does not help.

It would be highly appreciated if the carbon values would be extracted for the period selected and the low carbon energy would be calculated from the total.