BUG: Raise error when attempting to create dtype=object sparse matrix.
DOC: Add note about SciPy being for numeric data.
ENH: Add getrow and getcol to DOK.
BUG: Fix casting in _mul_scalar for DOK and LIL.

The way I did the casting is pretty hackish. There should be a better
way.
TST: Turn on scalar multiplication tests for LIL and DOK.
BUG: Remove broken take and split methods from DOK.
ENH: sparse: move scalar upcast dtype computation to a helper function
MAINT: sparse: convert object dtype error to a warning
Merge pull request #2719 from cowlicks/dok-lil-improvements

BUG: sparse: DOK and LIL improvements

* Raise a warning when you try to create a sparse matrix with
  dtype=object, and add a note to the docs stating that SciPy Sparse is for
  numeric data.
* Remove the broken and undocumented take and split methods from DOK, add getrow and getcol.
* Fix the casting for scalar multiplication with LIL and DOK and turn on tests for them.

Fixes gh-2528, gh-2645, and gh-2542.