Move to using index_parser in SLTev

[obo]

Vojtech's solution to #19 is soon to be finished. Once he merges his index_parser to the master, please make sure that regular SLTev uses the function and correctly processes the new indices.

Sync with ELITR/elitr-testset#21

[srdecny]

And while we are at it, it would be nice to make another pip release.