Abstract out integration of newly evaluated candidate into population

This is a requested feature.
The functionality is technically already available by including the desired behavior in the loss function, but they want that anyway.

_evaluate_individual should then call
```
_breed
loss_fn
_integrate
```
The user can call these themselves and the control flow returns to them.
_integrate should update the population with the freshly evaluated individual and send it out the the other workers on the same island.

We should probably ask for a concrete example to make sure the rest of the communication still works.

Start by tracing _evaluate_individual and the existing _breed, loss_fn, and _integrate flow. Clarify a concrete example and inspect how updates are communicated to workers on the same island; done means the steps can be called independently while preserving control flow and population synchronization.

Written by the indexing model from the issue text.