Model iteration setup

I initially had more than this, but pared it down after a conversation about the first iteration of this feature.

My other work is here: https://github.com/dimagi/Vellum/compare/hq-data-api
I know we have a few other modules with mixed case names, but my preference is to use all lowercase file names. What do you think of renaming this file to `src/modeliteration.js`?