ENH sparse: Add Ellipsis indexing to sparse matrices.
TST sparse: Add tests Ellipsis indexing with sparse matrices.
ENH: sparse: simpler way to expand the Ellipsis
Merge pull request #2759 from cowlicks/sparse-ellipsis-slicing

ENH: sparse: support Ellipsis in slicing

This add indexing with Ellipsis (...) to sparse matrices.

Fixes gh-2721