fixing #63
Adding Riechers citation
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Changed multiprocessing map to use large chunksize.
Better chunksize argument (doesn't leave one processor hanging) for multiprocessing.
Better choice of chunksize (length / threads, rounded down).
Revert "Better choice of chunksize (length / threads, rounded down)."

This reverts commit a5b5cdb7b33c26a3cbcdb2143da3d3db937a89b8.
Revert "Better chunksize argument (doesn't leave one processor hanging) for multiprocessing."

This reverts commit 9a15bb412b77109e84598b383d7ed4a692146b76.
Revert "Changed multiprocessing map to use large chunksize."

This reverts commit 15ebade2c5ba497f5f6e7a97cb7bbf70ed2d1936.
Re-wrote PT sampler to take more advantage of parallelism.
Merge branch 'master' of github.com:farr/emcee