Store and visualize iterations

[dilpath]

• store iteration in model YAML
• use this to make graph_iteration_layers nicer, i.e. each column is an iteration
  • also show "method" of each iteration, e.g. to make FAMoS visualization clearer. ideally things like "jump_to_most_distant" are also visualized... could be an additional iteration_method key in the model YAML.
• require a layout in graph_history, e.g. users provide their graph object generated by Add post--model-selection methods #110, and e.g. the output of spring_layout

Other small fixes

• make model labels consistent in visualization notebook (table at top currently has "old" labels)
• add iteration information to line_selected plot -- then plot best model of each iteration