Better Chunking for Multiprocessing Pool

Modified the uses of multiprocessing's map function to use a chunksize that is max(1, N/(Nthreads + 2)).  Having a larger chunksize tends to lower the overhead for pickling and unpickling the results of likelihood computations; in my tests this significantly (about 30%) improved the performance of multiprocessing on modest-length (100 ms) likelihood computations.  It shouldn't slow down any computations significantly.