Minor refactor/ cleanup.
Minor refactor/ cleanup.

* Internal `_downsample*` functions take the count matrix instead of the AnnData object
* Added caching to `_downsample_array` (speeds tests up a lot)
* Doc updates
* Made it so original sparse matrix type is returned for downsampling total counts
* Eliminate zeros after downsampling
* Minor miscellaneous cleanup/ formatting

theislab/scanpy,theislab/scanpy