Feature: add support for computing SurvSHAP using treeshap-algorithm

[kapsner]

The purpose of this formal feature request is mainly for a documentation of ongoing works and to avoid duplicate efforts.

I've started to work on implementing support for computing SurvSHAP values using the treeshap algorithm in this fork of survex.

As of now, both, local and global computation of SurvSHAP values using the ranger algorithm work technically.

Statistical / mathematical correctness needs yet to be proven for:

• [x] computation of predictions of the survival probability at different time points using treeshap
  • see this issue
  • related code is located here
  • one possibly required enhancement includes to use the stepfunction logic from here in the treeshap-package to apply it to the predictions from ranger in order to address that currently, only the time-points that are computed internally by ranger are used for computing the survival probabilities (see here), which results in this code in survex and which is some kind of a workaround currently
• [x] calculation of aggregated "global" survshap values for each time-point and feature across multiple observations
  • see this PR

[kapsner]

Regarding the computation of the prediction of the survival probability at predefined time points using treeshap, the latest code adaptions seem to work well: the SurvSHAP-plot computed with treeshap produces very similar results compared to the one computed with kernelshap (but computation with treeshap was faster).

Plot with Treeshap (code-ref https://github.com/kapsner/survex/blob/d2c16a608792c6387b2e65890446521890d088e0/tests/testthat/test-predict_parts.R#L122):

Plot with Kernelshap (code-ref https://github.com/kapsner/survex/blob/d2c16a608792c6387b2e65890446521890d088e0/tests/testthat/test-predict_parts.R#L137):

[mikolajsp]

Completed in #87