Allow for setting time slice level for availability constraints

[alexdewar]

We currently treat all availability constraints as being at the daynight time slice level. If the user specifies the constraint at a seasonal or annual level, it is assumed that the constraint applies to each individual time slice of the season or year. However, users may also want availability to be specified at a seasonal or annual level: e.g. 50% availability could be satisfied with 100% availability for half the year and 0% for the rest.

It's a bit counterintuitive that specifying a constraint in process_availabilities.csv with a time_slice of annual should lead to there being separate constraints for every time slice, but I think this makes sense: it's conceivable that users might want uniform availability at the time slice level without having to specify separate entries for every time slice.

I think the right way to do this is to have a separate time_slice_level column, as we have in commodities.csv. I'm not sure whether it should be specified at process level (i.e. in processes.csv) or at constraint level (process_availabilities.csv) though. Then we can treat the specified availability entries differently depending on the specified time slice level.