Feature custom validation set

[diego-s]

This pull request addresses Issue #25 adding support for custom validation sets by passing an array of integer validation set indices as the 'frac' argument instead of a scalar fraction value.

[RaphaelS1]

Thanks @diego-s I really appreciate the contribution. But a couple small points:

1. It doesn't make sense to me to have a parameter called frac that is passing indices. It would make more sense to have two separate parameters like frac and validation_indices (or any other name) where only one can be non-NULL
2. Please update the documentation to reflect the new parameters
3. I'd appreciate if you can extend this to dnnsurv
4. Please add tests to cover the new method

[codecov-commenter]

Codecov Report

Merging #28 (eaccf43) into main (7bca543) will decrease coverage by 0.12%. The diff coverage is 66.66%.

@@             Coverage Diff             @@
##              main      #28      +/-   ##
===========================================
- Coverage   100.00%   99.87%   -0.13%     
===========================================
  Files           16       16              
  Lines          768      770       +2     
===========================================
+ Hits           768      769       +1     
- Misses           0        1       +1     

Impacted Files	Coverage Δ
R/helpers_pycox.R	99.63% <66.66%> (-0.37%)	:arrow_down:

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