Support model formulas in workflow sets

[hfrick]

When constructing a single workflow, I can pass a "model formula" via the formula argument of add_model(). I'd love to be able to do that when constructing a workflow set. This matters for models that need a model formula, e.g., mixed effects models or stratified survival models.

[simonpcouch]

Ah, I just came across update_workflow_model() while making some other changes, which does provide an API to pass a model formula to workflow sets!

Maybe this could be better documented?