[POC | not ready for merge] Speed up bootstrap using multiprocessing

[rebecca-burwei]

Trying out some things I learned at SciPy.

Locally, I'm seeing a 4-5x speed-up on the tests. I wonder what the speed-up would be in our production environments.

I understand we're introducing a bit more non-determinism here, but I bet it's possible to make the difference small enough.

If this works, we should implement it for the Bayesian bootstrap too.

Other tips:

• This can probably be done with multithreading, which will use less memory and take less time.
• Another way to speed things up is to use numba on stat_fn's that are not just simple compositions of numpy functions.

Tagging some folks for feedback on this proof of concept.

[scholtzan]

how does #177 compare to this?

[mikewilli]

@rebecca-burwei Should this be closed?