Calculate demand for each time slice #231
Description
Demand is already assigned to the relevant Commodity struct, grouped by region, but it would also be good to be able to look it up based on time slice. In fact, we may get away without storing the annual demand in Commoditys at all: it might be sufficient to have some kind of a map from time slice + region to demand as a field instead.
To do this, we'll need to divide up the annual demand between each of the time slices, weighting according to the values of the "demand slices". Note that time slices are not all of equal length, so this will also need to be factored in.
Demand also varies by year, but I'm not clear about some of the details of that (see #230), so I'll mark this as "on hold" until that's resolved.