Allow calculating indicators on scenarios

I had the same thought -- it does seem like the base data could just be represented as a scenario. There are a couple special things with the base data: notably that it needs to have multiple sample periods, whereas all scenarios only have a single sample period. Plus there are some indicators that only should be calculated on the base data: the weekly service hours, and the forthcoming jobs indicator/overlay. So it's not trivial and would require a bit of reworking. I think it's probably not worth it, though we should keep it in mind for a potential refactor in the future.