[minor] Make the tool work for carbon intensity intervals of arbitrary sizes

[Llannelongue]

(just saving it as a small job for later if anyone has time)

For now, the tool assumes the carbon intensity forecasts are regular intervals throughout (e.g. every 30min). It would be good to either keep that assumption but check it by testing thee forecasts, or make the integration method work for any kind of intervals (which would be cleaner).
(discussed here briefly)