Refactoring multi-labels and adding attributes options

- Refactoring all multi-label resource's function to store and
  read the multi-label associated labels and fields' variables
  in a user_metadata attribute of each created resource

- Adding --source-attribute, --ensemble-attribute,
  --evaluation-attribute  and --batch-prediction-attribute
  to point to JSON files where attributes for creation function
  calls are specified
Refactoring the main routine and other pep8 and pylint cleaning
Adding missing arguments file
Adding new --batch-prediction-tag to docs and HISTORY
Changing variable to constant in labels processing