Greg Martin

This actually doesn't go far enough. `pmap` still gets passed a reference to the chain object, which is growing over time. Will update shortly to fix that.

This change by itself doesn't give much of a memory improvement. To get any improvement it's necessary to use `CachingPool`. I couldn't figure out a way to do this cleanly....

I've added an option to AlgoBGP.opts, "worker_pool", that allows the user to specify a worker pool to use for distributed computation. This gets passed to the `pmap` step in `computeNextIteration!`....