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
DOC: Fix typo in signal.welch example

The example did not run since `1024` is not a valid window name.  Since
it didn't run, no figures were displayed in the online docs.

Also added calls to plt.ylim so the figures look reasonable in the docs.
S
Merge pull request #2856 from ewmoore/welch-example

DOC: Fix typo in signal.welch example