Add option to display observed data as points in ppc_km_overlay() and ppc_km_overlay_grouped()

[Sakuski]

Fixes #558

Declaration of AI Usage

AI-based tools were used to help to create some parts of the code. All AI modifications were carefully reviewed manually.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.75%. Comparing base (e651a2a) to head (66e42c6).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #560   +/-   ##
=======================================
  Coverage   98.75%   98.75%           
=======================================
  Files          36       36           
  Lines        6966     7014   +48     
=======================================
+ Hits         6879     6927   +48     
  Misses         87       87           

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[jgabry]

Looks good, thanks for the PR!

R cmd check failed only because it needed the .Rd file regenerated after adding the new argument. I made a couple of very small edits and will merge this when CI passes. Thanks again.

[jgabry]

@Sakuski I forgot to ask you to do it as part of this PR, but if you want can you make a new PR adding yourself as a contributor in the DESCRIPTION file? I think this is your third helpful PR, so happy to add you if you want.