Clean up multiprocessing.Pool after use in _map_parallel

The pool created for multiprocessing in model._map_parallel() was not
being cleaned up after use, which could potentially leave lots of
abandoned Python processes and unclosed files.
Clean up multiprocessing.Pool after use in _map_parallel

The pool created for multiprocessing in model._map_parallel() was not
being cleaned up after use, which could potentially leave lots of
abandoned Python processes and unclosed files.

The Python 3 map function returns an iterable, not a list.  I've put the
list() call back around the non-parallel map.
Merge pull request #56 from ToddSmall/fix-multi

Clean up multiprocessing.Pool after use in _map_parallel