getNestedTuneResultsOptPathDf: Get the opt.paths from each tuning step from the outer resampling.
Description
After you resampled a tuning wrapper (see makeTuneWrapper)
with resample(..., extract = getTuneresult) this helper returns a data.frame with
with all opt.paths combined by rbind.
An additional column iter indicates to what resampling iteration the row belongs.