Change (x, y) from tuple to list for consistency
Add dispersal
Add birth process

Birth is the only association-dependent process in current model. Its
effect on birth rate of species i is controlled by two parameters - the
summed association size Ai from all individuals of other species
(weighted sum based on matrix A), and association strength k. The
functional relationship between the parameters and birth rate bi, though
arbitrarily defined, ensures that b = b0 when k = 1 (when association
doesn't matter), and b = b0 when Ai = 0 (when net association is zero).
When Ai = Amax, b = b0 ** (1/k). When Ai = Amin, b = b0 ** k.