PermutationImportance uses CV splitter indexes incorrectly

[zeromh]

This version of PermutationImportance still seems to have the same (serious) bug that I filed at the other org 4 years ago:
TeamHG-Memex/eli5#411

[ClaudioSalvatoreArcidiacono]

@lopuhin I would like to help to fix this issue, will you have some time to review a PR if I submit one :)?