Decouple all subsectors as much as possible

[brynpickering]

If you don't want to model a particular subsector, only a minimal set of data processing related to that subsector should be undertaken. At the moment the rules annual_subnational_demand and update_electricity_with_other_sectors are the most troublesome w.r.t. decoupling, since expect all sectors to be involved. The reason for current coupling is two-fold:

1. Similar datasets and spatial operations are used to disaggregate national data to subnational units; separating them might lead to lots of time spent doing the same operation multiple times.
2. Each subsector uses some amount of electricity at present, which is removed from the OPSD electricity demand profile. This is undertaken all in one go at the moment, since they all touch the same output file.