Replace some maps with `TimeSliceID`s as keys in agent investment code with simple `Vec`s

(The `Vec`s should be ordered by `TimeSliceID`, obviously!)

I think this will make the code simpler and will also be much easier for the compiler to optimise. We don't care about performance that much at this stage, but it seems sensible to try to get the design right sooner rather than later.

As part of this I think we should change various flow/demand maps to be something like `HashMap<(CommodityID, RegionID>, Vec>` instead, so we can get the values for all time slices in one step.