Refactor input validation.
Merge pull request #3443 from amueller/input_validation_refactoring

[MRG] Input validation refactoring
remove check_arrays stuff and old input validation
ENH add allowed_sparse named argument for @ogrisel
Merge pull request #3447 from amueller/input_validation_b

[WIP] remove check_arrays stuff and old input validation
FIX classes name in OvR
DOC mention doc-building dependency on Pillow
ENH Sparse multilabel target support

This introduces a series of helper classes that abstract away
aggregation over multilabel structures. This enables efficient
calculation in sparse and dense binary indicator matrices,
while maintaining support for the deprecated sequences format.
ENH multilabel metrics now calculated over sparse indicators
STY remove unused imports
COSMIT intrinsic documentation of matrix operations
COSMIT roll _multilabel_to_csr into _check_clf_targets
COSMIT rename _check_clf_targets to _check_targets
COSMIT remove unnecessary label_order parameter