Chunk computations out into a dispatch queue

[ghost]

Cocoa provides an intelligent framework for parallel processing called dispatch queues, which is so easy to use that we'd be silly not to take advantage of it. At the very least, moving model computations into a queue would free up the UI to respond to events during computation processing.

There is a caveat: many default Cocoa collection types (including NSMutableArray) are not thread-safe. This probably puts a damper on how much parallelism we can actually expect, beyond making the UI more responsive.