Modify immigration_ver2 to be consistent with Bell

- Bell's model assume a Bernoulli trial with prob m for one ind from
each sp to immigrate to one cell per generation.
- Thus the total number of immigrants per cell per generation is
binom(S, m).
- In my model is prob is modified with respect to the global rad.
- The total number of immigrants per cell per generation is still
binom(S, m).
- Given total # of immigrants N ~ binom(S, m), # of immigrants belonging
to sp i is then binom(N, sad(y)).
- Combining the two yields n_i ~ binom(S, m * sad(y)).