Keep Dask fragments out of commits

[accurrently]

The folder dask-worker-space/ is a cache folder (like __pycache__/) for projects that implement Dask. If a data science or distributed computing project package uses Jupyter notebooks (e.g. in an examples/ folder), there is a chance that they might use Dask. Similarly, tests for modules that implement Dask will create a dask-worker-space/ folder as well if the code is run. This addition keeps that cache out of commits.

See Dask on Github repo for project information.