Add option for non-stratified sampling in linear_pool_sample()

[lshandross]

If we do something like this, maybe we should introduce an argument like method allowing to specify how to take the samples: "stratified" (the default?) for stratified by model, "random" for random across models, possibly with other methods to be added in the future.

Originally by @elray1 in 27: "handle sample types in linear_pool"

[nickreich]

Is this a near-term TODO or a wish-list item?

[lshandross]

Probably more of a wishlist thing