Use Set[Tuple[int, str] instead of List[Dict] in all dealings with "state"

[willu47]

smif state consists of an intervention name and build_year pair.

At present, this the store reads and writes to and from a dict [{'name': , 'build_year'}, ...]
This could be replaced with a list or set of tuples

• Replace build_year with build_timestep
• Replace list of dict with list of tuples