Use `TimeSliceSelection` for demand maps

Rather than storing demands at the `TimeSliceID` level, which requires some hacky steps to split up demands for seasonally balanced commodities then add then together again, we should store demands at the `TimeSliceSelection` level

I had a go at this in #1329, and pretty much got it all working, but closed that as it's now got a lot of merge conflicts, so easier to start again

Start by reviewing the closed attempt in #1329 and locating the current demand-map code and TimeSliceSelection usage. Done means demand maps store demands at the TimeSliceSelection level without the seasonal split-and-recombine workaround, while preserving existing demand-map behavior.

Written by the indexing model from the issue text.