Stratification based on censored data

[hfrick]

We currently don't allow stratification based on Surv objects and instead suggest to use the time or event variable directly:

rsample/R/misc.R

Lines 105 to 107 in 46de181

	if (inherits(data[, strata], "Surv")) {
	rlang::abort("`strata` cannot be a `Surv` object. Use the time or event variable directly.")
	}

Given that we've settled on the advice to make the Surv object outside of a recipe, people may not have the individual variables available. Soo we should rethink this.