Umis

- Enable cb_histogram to be used on samples without UMIs.
- Enable filtering of cells during `demultiplex_cells`.

- Python 3 support

- Add `demultiplex_cells` subcommand to break a transformed FASTQ file into separate FASTQ files by cell.
- Future proofing for changes to panda's `to_csv` function.

- Add support for click 7.0.

- Fix for min-length filtering with paired samples. Previously required only one read to be longer, fix requires both.
- Fix tests for fastqtagcount to use indexed BAM files.
- Support gzipped cellular barcode files.
- Support 10x V2 barcoding scheme. Thanks to tomasgomes for the fix.
- Re-enable streaming for cellular barcode filtering.
- Add `--umi_matrix` option to fasttagcount. This outputs a non-umi-deduped matrix of counts, useful for QC.
- Support gzipped files for `sb_filter`, `mb_filter` and `add_uid`.

- Fix `fasttagcount` off-by-one issue.
- Add `version` subcommand.
- Fix missing pandas import in `sparse` subcommand.