Fixed bug with "keep_samp_for_vS"… (#349)

* Fixed bug with "keep_samp_for_vS" in the internal function postprocess_vS_list() which occured if keep_samp_for_vS = TRUE in the explain() function. Previously, the code did not take into consideration that vS_list became a list of lists when keep_samp_for_vS = TRUE and therfore extracted incorrect names.

* add test

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Co-authored-by: Martin <jullum@nr.no>