Make uprating growthfactors top-down explicit

[nikhilwoodruff]

We should specify these at the outset. Proposing more as a general idea, open to feedback.

[nikhilwoodruff]

Have thought about this. I propose we move all economic growth factors to a parameters.economy folder and have all of these be YoY growth factors. Then, in the tax benefit system initialisation, we will take these (post any reform) parameters and generate all the other derived parameter indices (e.g. the triple lock, etc) in one go rather than how it's inconsistent now.