Two new methods

- Add culling_v2 where culling is a binomial process with carrying
capacity K as a "soft" bound (i.e., an expectation that can be
occasionally exceeded), which is much faster than the previous version.
- Add proc_comb where the five processes (immigration, birth, death,
dispersal, culling) are all wrapped up together, so that the loop goes
through each species in each local cell once in each time step, instead
of four times. However there is hardly any speed gain.