Loop construction of multiple cohorts

Loop the generation of code to create multiple cohorts. Grade-specific outcomes should be generated in other tasks, and be internal to this loop.

Start by locating the existing code that generates a cohort and identify how grade-specific outcomes are currently handled. Define the loop boundary so multiple cohorts can be generated while those outcomes remain internal to it; done means the requested multi-cohort construction works without changing the separate grade-specific tasks.

Written by the indexing model from the issue text.