Expose setters for island selection and replacement policies

Getting list of islands would already be helpful, because as it stands now, one has to index each island directly or loop over the whole archipelago if one is interested at a specific part of it.

Useful for a migration study (given a specific topology) would be for example:
```
sending_islands = archi[:k]
receiving_islands = archi[k:]

for isl in sending_islands:
isl.set_s_policy(...)

for isl in receiving_islands:
isl.set_r_policy(...)
```
While those setters still don't exist and constitute probably another issue...

_Originally posted by @CoolRunning in https://github.com/esa/pygmo2/issues/5#issuecomment-581437594_