Added basic matrix vector product examples
See #1531
Hello :)
This is actually the most embarrasing commit to do, but here
it goes.
ENH sparse: Allow sparse matrices to be size 0.
TST sparse: Test arithmetic & slicing with size zero sparse matrices.
DEP: linalg: deprecate expm2 and expm3

The Pade approximant provided by expm is a superior alternative, so
providing two worse methods just clutters the documentation.
DOC: linalg: explain the Taylor order used in expm3

Better to fix this in the documentation than to change the definition.

Fixes gh-2212
DOC: update release notes on deprecations
DOC: release: update release notes vs. LinearOperator changes
Merge pull request #2738 from pv/deprecate-expm23

 DEP: linalg: deprecate expm2 and expm3
TST: sparse: more zero-size matrix tests
Merge pull request #2755 from cowlicks/size-0-matrices

ENH: sparse: Size zero sparse matrices

This allows sparse matrices to be size zero. Meaning one or both of their shape
components is == 0.  This is needed because fancy indexing and other weird edge
cases occasionally need to return a size-0 sparse matrix.
Merge pull request #2745 from Juanlu001/sparse-docs

DOC: sparse: Added basic sparse matrix vector product examples