tidymodels updates

[topepo]

We've made some changes that should propagate to these materials. The main two changes are:

• Every model now has a default engine. For example, lm() can be fit specified without calling set_engine().
• We've deprecated the pull_*() functions with a simpler set of extract_*() functions.

I've forked the repo and I can do a PR if you like. I'm not sure how to handle the default engine bits in the notes since the slides use engine setting for lm() as an example.

The pull -> extract changes are pretty trivial though.