0035 pre model gqs

[bob-carpenter]

This is ready to review.

I tried to flesh out how we'd actually code this and document it in the Reference Manual.

[WardBrian]

Rendered: https://github.com/stan-dev/design-docs/blob/0035-pre-model-gqs/designs/0035-pre-model-gqs.md

[bob-carpenter]

@WardBrian and @SteveBronder and @nhuurre:

I think I've addressed all the issues in this if you'd like to take another look.

Thanks.