Handle pending steps in the scheduler
Currently iota2's scheduler assume that the dependency graph is ending with only one step (actually the last step append in the list of steps). For some needs it could be convenient to gather pending steps too.
A "simple" workflow could be "among all tasks, retrieve all the tasks that are not dependent on another task and add them to a fake gathering task".
I have no idea how much development/refactoring effort this feature requires.