Bug fixing: evaluations for --dataset and --number-of-models > 1

- the evaluation did not select the 20% hold out instances to test
  the generated ensemble
First version of multilabels with ensembles
Bug fixing: retrieving models info when resume starts in ensembles' creation
Adding --ensembles and --ensemble-tag to retrieve previously created ensembles
Merge conflicts
Docs and new tests for multi-label ensembles