Model heterogeneous migration

[isaacovercast]

Right now we have one colonization rate for all meta-community species. This is kind of implicitly modeling heterogeneous migration rates bcz meta-community species will colonize proportional to their frequency. But this doesn't track Ne properly. If we wanted to actually test heterogeneous migration we'd need to actually implement some kind of mechanism, maybe each species would get a migration rate then you could average over all of them to get proportional migration rates. This is a V2 feature.