Refactor commodity cost time slices

[alexdewar]

Time slices for commodity costs can be specified as a single time slice, a whole season or the whole year. There may be contradictions though, e.g. if the user specifies the same time slice twice or adds a specific time slice when the whole year/season is already covered. We should check for this when loading data.

Edit: Similarly to demand slices, we will also need to refactor the commodity cost code because currently the values are not particularly easy to use. It would be useful to have a data structure which can be used to easily look up the cost for a given region + year + time slice combo.

[alexdewar]

I've realised this pertains to demand slicing too, so I'll have a go at this while the demand stuff is on hold.

[alexdewar]

...although I've just realised we have the same problem as for the demand stuff in that really it would be best to make a table of year/timeslice vs. cost for each commodity, but this depends on me actually understanding how milestone years work.

On hold until #242 is resolved.