Add dask as dependency

Realized that it's not listed as a dependency even though it was being used in some of the code. This is needed when creating the environments for each proposal.

(On that note, one can now use scs-create-toolbox-env --proposal <PROPOSAL> to generate the environment.)

@lleguy @mercadil

doc/maintainers.rst

@@ -76,7 +76,7 @@ run the following commands:
     source envs/toolbox_<IDENTIFIER>/bin/activate
     pip install ...
 
-There's no need to install the ``exfel`` module.
+There's no need to load the ``exfel`` module.
 
 
 Updating the source codes