[k88hudson-cfa]

This replaces the EdgeType network API with layered networks. A network is declared once with a set of layers (household, school, workplace, etc.). Every method takes a layer value (or in the case where you don't define layers, the name of the network):

define_network!(Contact, Person, layers = { Household, School }, data = f64);
define_network!(InfectionTree, Person); // one layer, no edge data

context.add_edge_bidi(Contact::Household, p1, p2, 1.0)?;
for edge in context.get_edges(Contact::School, p1) { /* edge.neighbor, edge.data */ }
let contact = context.sample_edge_weighted(TransmissionRng, Contact::School, p1, |e| e.data);