Separate pickling of variables and parameters

[evan-magnusson]

Make a pickle just for the exogenous parameters, and another for just endogenous variables. There is currently one for parameters, and another for parameters and endogenous variables. Remove the overlap.

[evan-magnusson]

Finished. Deleted all parameters from SS and TPI pickles, and just use params_given.pkl to store parameter values. params_changed.pkl stores parameters that have been changed for tax experiments separately.