Add "stand alone" scripts as an output

[ajmedford]

It should be possible to have the model output a "stand alone" script after it runs. This script would be capable of reading in the output as an "initial guess" and either re-solving the model with a more serial workflow or at least checking the residuals to provide a sanity check. If one sets the attribute "create_standalone = True" then an experimental "stand_alone.py" file will be output which contains all of the auto-generated functions. In principle this could be combined with something similar to the "simplekinetics" module in order to create a full stand alone.