Change "nu" parameter to "xi_ss" and "xi_tpi"

[rickecon]

I noticed that we have one single parameter for the convergence rate of functional iteration "nu" in default_parameters.json. In our documentation, we have two separate parameters, xi_ss and xi_tpi. I recommend that we break the current "nu" parameter into two separate parameters xi_ss and xi_tpi. This will entail changes to default_parameters.json, SS.py, TPI.py, some tests, and some pickle files. @jdebacker