Bug fixing: extending input data in multi-label predictions

- For multi-label models, the input data multi-label fields are now
  extended into the new label fields.

- For user-given labels in multi-label models, the objective field
  labels are now restricted to the ones given by the user and they
  are stored in the user_metadata structure.
Updating version to 1.4.1 and logging changes in HISTORY.rst
Cleaning with pep8 and pylint help
Bug fixing: retrieving all the model fields for random forests to predict