Add documentation to `tmp_dir` in parallelization

[HaleySchuhl]

Start a discussion about anything you would like The tmp_dir part of a config value defaults to None. We have instances of example config files but it's not abundantly clear that it's needed for the parallelization workflow to print out and gather intermediate json files with data.

[nfahlgren]

tmp_dir is not a required input, it can be left as None. When you run plantcv-workflow.py and tmp_dir is set to None it uses the tempfile package to make a temporary directory. The config setting just lets a user pick the location instead of it being chosen by the system.