Allow group to execute action before terminaison
Work needs to be done before declaring wanting to transit between running and done states. This work is, asking the underlying transport to kill its instances and clean stuff on the remote computers. Thus, declaring that the work is done is wrong. The work is done when this step is done. To enable this asynchonous possibility a task can now ask the execution of a callback function when a changing state is possible. It's a way to execute an out-of-workflow operation still inside the state-machine transitions.