ENH: solution_path method for computing solution path of regularized fits

This PR adds a method called "solution_path" to base.LikelihoodModel.  The solution path is the set of parameter estimates obtained as the penalty weight parameter is varied.  The method works with any model that has a "fit_regularized" method.  It calls that method repeatedly to construct the solution path, using bisection to focus computational effort on the places where parameter estimates become zero.