[FEATURE] save&load (e.g., pickle) population snapshot easily based on certain criteria

Hi,

i couldnt find any example to be able to save&load the population during evolving.

For example, something like this would be really useful:

# pseudo code:
```
for i in range(n_iterations):
pop = evolve(pop)
if pop.champion_f < best_f:
save_snapshot(pop, i)
```

I wonder how / if this wold be easily possible with pygmo. Any example would be highly appreciated!

P.S: Would this way of evolving break any adaptive DE variants ?
Many Thanks