Improve numerical accuracy of confint(::KaplanMeier)

[ararslan]

This works in the case(s) exercised in the tests but could do with some more edge case testing/comparison against other software.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.32%. Comparing base (646828a) to head (5339f9f).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #57   +/-   ##
=======================================
  Coverage   99.32%   99.32%           
=======================================
  Files           6        6           
  Lines         297      297           
=======================================
  Hits          295      295           
  Misses          2        2           

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