Combine recruitment and init_cohorts subroutines into one subroutine

[adrifoster]

Right now recruitment and init_cohorts essentially do the same thing (initialize new cohorts), so most of the code is exactly the same, but with some small differences (accounting for SP mode, etc.)

We should make these one subroutine so that we don't duplicate code and so that we can ensure they function the same way if we want to update anything.