def nodes(self, nodeType=None):
        returnList = list()
        for dagNode in self.network:
            if not nodeType:
                returnList.append(dagNode)
            elif dagNode is nodeType:
                returnList.append(dagNode)
        return returnList
Return a list of all nodes, nodes of a certain type, or nodes that have 
dataset/ETHPy150Open andrew-gardner/dependsworkflow/depends/depends_dag.py/DAG.nodes