improve discrete generational network add_node method
improve discrete generational network add_node method

berkeley-cocosci/Wallace,Dallinger/Dallinger,jcpeterson/Dallinger,Dallinger/Dallinger,Dallinger/Dallinger,jcpeterson/Dallinger,Dallinger/Dallinger,suchow/Wallace,jcpeterson/Dallinger,jcpeterson/Dallinger,Dallinger/Dallinger,suchow/Wallace,jcpeterson/Dallinger,berkeley-cocosci/Wallace,berkeley-cocosci/Wallace,suchow/Wallace