Add nodelist option for SLURM
 Preparing modifications in the treatment of dependecies.
 Use pydispatch to implement the observer design pattern in ABINIT workflows.
 Changed the interface of Workflow so that workdir and manager are
 optional arguments. Their value can be set outside __init__ with setter methods.
 Refactoring the treatment of dependencies in Task and Workflow
 Use node_id to implement __eq__ and __hash__