feature: rate scaled SRM branches

[chrisdicaprio]

We want to apply scaling of rupture rates in toshi-hazard-post rather than have OpenQuake explicitly calculate those branches

• Feature: rate scaling for branches nzshm-model#21
• apply scaling when loading data so that number of multiplication operations is minimised (at the cost of memory)
• Value store requires new lookup dimension for scaling (along with solution id, realization number, location, and IMT)