Consider defining additional generics for a graph

[EeethB]

Throughout other functions that utilize graphs, it's common to be calling length(graph$hypotheses) or dimnames(graph$transitions), etc. So here are a few generics that might be useful to define differently for a graph

• length.initial_graph as length(graph$hypotheses)
• names.initial_graph as names(graph$hypotheses)
• dimnames.initial_graph as dimnames(graph$transitions)
• str.initial_graph with prettier printing of some sort? Need to be careful with this one, since it's so useful for exploring underlying structure outside of printing
• seq.initial_graph as seq_along(graph$hypotheses)
• ...Others?