Rename to match reality

The current data structure doesn't describe the actual domain, its StateAidCalculation is actually a PremiumSchedule so I've renamed it (and had to rename the existing PremiumSchedule).

Renamed PremiumSchedule -> PremiumScheduleGenerator
Renamed StateAidCalculation -> PremiumSchedule

PremiumScheduleGenerator is a bit of a stop-gap name as this is an area we are working on at the moment and will probably soon change.

None of this changes the workings of the app (just a few URLs) so it could be merged now, however I'm not sure if it should. Either way, the current premium schedule work we're doing will follow on and build from this.

Discuss.