Kb-python

`ref`
- `kb` now provides a pre-built human index for RNA velocity (`linnarsson`)
- The intronic fasta with the `--lamanno` option now includes 30-base flanking regions.

`count`
- Unfiltered count matrices will always be placed in the `counts_unfiltered` folder.
- If the `--filter` option is specified, the filtered count matrices will be placed in the `counts_filtered` folder.

Official release.

`count`
- `-o` is now optional. Defaults to current directory.

Documentation
- Developer docs hosted on Read the Docs.

Official `PyPi` release.

`count`
- New `--filter` option. This option generates a filtered count matrix. It can not be used with `--lamanno`. Currently, this option only supports filtering with `bustools`.

Fix for python 3.6