add gen_additive_mod

[mdancho84]

Hey @topepo et al,
Here is the work-in-progress version of the gen_additive_mod() function. I'm OK with changing anything, I'd just like to see GAM's get into parsnip.

[topepo]

Thanks for send this in.

Do we want to have gam as an engine? They frequently update the package to the most recent version of R and make breaking changes (without much developer sympathy/consideration). At the minimum, I'd like to make the default engine mgcv.

We're almost done with a new documentation system for parsnip and I've adjusted the doc file (incompletely) to prepare for this. (I'll do a modeltime PR to show how it is done; it is opt-in).

[mdancho84]

Thanks for send this in.

Do we want to have gam as an engine? They frequently update the package to the most recent version of R and make breaking changes (without much developer sympathy/consideration). At the minimum, I'd like to make the default engine mgcv.

We're almost done with a new documentation system for parsnip and I've adjusted the doc file (incompletely) to prepare for this. (I'll do a modeltime PR to show how it is done; it is opt-in).

Hey, I'll leave this one up to you. mgcv::gam() is the function that is being used, so maybe mgcv is fine.

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