This implements the basic Lotka-Volterra equations:
with being the reproduction rate and being the interspecies interaction. This is implemented as a single line:
density += repro_rate * density + (interaction * density) * density;
This command also increments the timestep counter tstep.
An optional argument specifies a number of timesteps to run the generate step. This improves the speed by amortising the real to integer conversion operation over a number of timesteps. The downside is that computation may fail if the problem is ill-conditioned (offdiagonal elements of too large with respect to the diagonal elements).