Kaplan-Meier confidence bands

[tbeason]

I think it would be good to add the ability to produce confidence bands in addition to the pointwise confidence intervals, at least for the Kaplan-Meier method.

The SAS PROC LIFETEST documentation is pretty thorough here (discussion starting ~pg 5155). There are some packages elsewhere that have pre-computed the appropriate critical values for some of the methods, so that legwork is already out there.

This would probably be a good first issue for someone.

[ayushpatnaikgit]

https://juliastats.org/Survival.jl/latest/km/#StatsAPI.confint-Tuple{KaplanMeier}

[tbeason]

Simultaneous confidence bands are not the same as pointwise confidence intervals.