cellranger_count.py: remove any existing output directory

[nick-youngblut]

Description of feature

For the sake of debugging failed CELLRANGER_COUNT processes, it would be helpful if cellranger_count.py removed any existing output directory so that .command.run can be run multiple times without having to delete the output directory in between each run (otherwise, you get the error RuntimeError: pipestance 'OUTPUT_DIRECTORY' already exists).

[grst]

Don't have any objections, but why would you need to do that? If a process fails it should start in a new work directory the next time.

[nick-youngblut]

For debugging: going to that working directory and re-running the job (bash .command.run). Definitely just a nice-to-have.

[grst]

Feel free to submit a PR to the modules repo!