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 :white_check_mark:

Project coverage is 97.18%. Comparing base (cb9bb8c) to head (f1bf90e).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #57      +/-   ##
==========================================
- Coverage   99.65%   97.18%   -2.47%     
==========================================
  Files           6        6              
  Lines         286      284       -2     
==========================================
- Hits          285      276       -9     
- Misses          1        8       +7     

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