Task callbacks

[sylvinus]

Have a task called with the results of a group of task when they finish (success/failed?)

[sylvinus]

This won't be in the 1.0 API, left to the users to implement for now. Maybe add a cookbook in the doc until then?

[sylvinus]

Have a look at https://github.com/openstack/taskflow

[sylvinus]

Also https://airflow.apache.org/