Modified argmax() method of
    * MultilabelModel
    * HashedMultilabelModel
to give correct value of ret->score
Merge branch 'develop' of https://github.com/shogun-toolbox/shogun into hashed-mutlilabel
* Minor bug fix in MultilabelModel.
* Added multilabel_structured_prediction notebook.
cleared all outputs of the multilabel_structured_prediction notebook
Merge pull request #2358 from abinashpanda/develop

Added multi-label structured prediction notebook