Disallow unseedable random sample methods through API

[greenape]

Only the random sampling methods which can be seeded should be available via the API, because things returned by the API are cached.

And with this, we should change the default random sampling method to a seedable one.

[jc-harrison]

Given that random_ids is the only seedable sampling method guaranteed to provide the requested sample size, I would suggest this is a good default. I suspect (but haven't checked) that this is the slowest method, but this is fine as long as the docstring explains the trade-offs and benefits of choosing a non-default sampling method.