ENH: Add log rank testing

[kshedden]

This PR adds log rank testing for comparing two or more fitted survival functions (fit with the Kaplan Meier estimator).

The unit tests are based on results from R and Stata.

[codecov-commenter]

Codecov Report

Merging #36 (4d40ac1) into master (755433a) will decrease coverage by 0.87%.
The diff coverage is 97.41%.

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
- Coverage   99.54%   98.67%   -0.88%     
==========================================
  Files           6        6              
  Lines         222      377     +155     
==========================================
+ Hits          221      372     +151     
- Misses          1        5       +4     

Impacted Files	Coverage Δ
src/Survival.jl	100.00% <ø> (ø)
src/kaplanmeier.jl	97.59% <97.41%> (-2.41%)	⬇️

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[lauswullum]

Great.
I definitely think Survival.jl should have log rank testing implemented.
We highly need this for Julia to be a viable option for doing statistics/bio-statistics.

[ararslan]

Yes, for sure, adding the log-rank test to this package has been on my backlog of desired features since about 2017. 😄 Apologies, I should have spoken up on this PR sooner; I haven't been ignoring it. I have a number of internal changes planned that should be able to significantly simplify the implementation and I've finally gotten around to making some of them. Once that's in a good state I'll help adapt this PR accordingly. Thanks for the contribution and for your patience, @kshedden!

[kshedden]

Great, thanks for the update. Also, it would be very useful to have an EntryTime field in the EventTime struct to be able to handle delayed entry/left truncation.

[casasgomezuribarri]

Is log-rank testing expected to be available soon-ish?