Use multi processing in model code

[gavinmacaulay]

Structure the model code to facilitate parallel processing, especially for combinations of input parameter arrays (#3).

Some models take a while to run (e.g., PSMS) and being able to divide the model runs across all CPU's would be advantageous.

[gavinmacaulay]

A trial implementation of multiprocessing is in the code

[gavinmacaulay]

The preliminary multiprocessing option isn't reliable - consider using a different multiprocessing library

[gavinmacaulay]

As of release 0.8.2, multiprocessing is well-documented and tested.