Multi-thread simulations

[lime-green]

As a user, the site should work quickly. Most time using the site is spent waiting for the sim to run iterations. We should profile and investigate areas to make this faster. Some thorough steps to document the process would also be helpful so this can easily be repeatable.

One obvious low-hanging fruit is parallelizing the sim, which as far as I'm aware is only done for stat weights but normal sims are done sequentially.