Only one Population Hashing Strategy Available

[drassaby]

Describe the bug
While the population can accept multiple different hashing strategies in its constructor, it's not possible to construct this using our islands/island builders yet.

Expected behavior
It should be possible to select the populations hashing strategy by users who do not create custom islands.

Additional context
We could pass in optional values for the hashing strategy all the way down and then default at the population level, but I think it would be better to set defaults for the hashing strategy at the island builder layer, and pass down the defaults all the way.