sh_island_mig_mat: return the migration matrix for a simple hierarchical island...
In eriqande/GSImulator:

This provides the migration matrix for one of the simplest possible island models with migration rates within and between groups.

1	sh_island_mig_mat(ru, Min, Mbt)

`ru`	a vector giving the numbers of collections within each group. For example, if there are 10 collections total and the first two are in one group, the next three are in another and the final five are in a third, then you would want `ru = c(2, 3, 5)`.
`Min`	the M = 4Nem value desired within groups. This will get scaled by the number of collections within the group (Min / (N - 1)) where N is the number of collections in the group.
`Mbt`	the M = 4Nem value desired between collections that are in different groups. This will not be scaled by the number of collections not in the group.