Mutlilabel data is indicated by having y be a 2d array consisting of 0s and 1s. This may be represented with a sparse matrix format. However, it seems we are lacking in tests of sparse representations of multilabel formats.

We'd like to see one or both of:

• specific tests for each multilabel estimator (or confirmation that they already exist)
• a general test for multilabel estimators that they support sparse y and behave identically to dense y
• clarity in the estimator docstrings where sparse y is supported.