Sparse DOK and LIL improvements

This fixes issues [#2528](https://github.com/scipy/scipy/issues/2542), [#2645](https://github.com/scipy/scipy/issues/2645), and [#2542](https://github.com/scipy/scipy/issues/2542).

* #2542 This raises an error when you try to create a sparse matrix with `dtype=object`, and adds a note to the docs stating that SciPy Sparse is for numeric data.
* #2645 This removes the broken and undocumented `take` and `split` methods. And adds `getrow` and `getcol`. 
* #2542 This fixes the casting for scalar multiplication with LIL and DOK and turns on tests for them.

The way I did the casting to correct #2542 is pretty hackish. Please let me know if there is a better way. See [these lines](https://github.com/cowlicks/scipy/compare/dok-lil-improvements?expand=1#L2R384).