Add missing dask to runtime dependencies

[aidanjgriffiths]

Tasks

• Add dask (optional: pinned version) to core dependencies

[tennlee]

I don't think dask is a core dependency. It's tested for in development, but isn't a requirement for the users unless I've overlooked it

[aidanjgriffiths]

Sorry, I think I maybe either misled or there is a bug in the pip install scores[all] command as I was getting ModuleNotFoundError's missing dask this morning.

[aidanjgriffiths]

After some local testing it seems that the pypi scores package isn't recent enough to contain the dask dependency in pyproject.toml. This will be fixed in the new pypi release so I will close this issue. The behaviour works fine when running locally e.g. `pip install .[all]