Issue 7, inseparable parameter in assing.folds/data.splitter/plm.trainer

Closes #7 (closed)

Re-code the functions assing.folds/data.splitter/plm.trainer a bit, so that the parameters inseparable and meta should be usable now. Secondly, implemented support for non-stratified CV schemes. LOO-CV, e.g. would have failed before, since the functions tested if all class labels are present in all folds (happening now only for stratified CV).

Additionally, check if Closes #7 really closes Issue number 7 and automatically also deletes the branch after merging, as said in the GitLab documentation 😃