Provides all active model state sizes from the network at the specified time step, output to a list of vectors.
prevalence.net(dat, at)Master list object containing a networkDynamic object and other
initialization information passed from netsim.
Current time step.
This network utility is used during the netsim simulation
process to efficiently query the current size of each state or compartment
in the model at any given timestep. For a two-group network, the current state
size for each group and overall is provided.